Last week was rather an exciting one for markets.

The prime minister gave us a deadline for Brexit. Upon hearing all the tough talk at the Conservative party conference, the pound took fright, and ran for the hills (or rather, the valleys).

So what’s next for sterling, and what does it mean for your investments?

What lies beneath the sterling flash crash

The biggest news of last week was the “flash crash” in the pound. On Friday morning, the pound plunged by 6% (a huge move for a currency) to below $1.19. It jumped back up, but still closed a lot lower on the day, at around $1.24.

There’s no explicit definition of a “flash crash”, but it describes a situation where an asset slides (or soars) by a far greater amount than usual in a short space of time, for no obvious reason.

Often “fat finger” trades (where someone puts the decimal point in the wrong place) or computers (“algos”, as they’re called) going off into a frenzy of selling, get the blame.

So how did it happen this time? The Financial Times makes the point that the pound isn’t the only currency this has happened to in recent years.

The New Zealand dollar (the kiwi) tanked against the US dollar during last August’s China panic (remember that particular panic? It came just after the Chinese devalued the yuan). And the South African rand took just 15 minutes to slide by a whopping 9% against the dollar in January this year.

This is partly down to banking regulations. As the FT puts it: “Tough capital requirements are making it more expensive to hold customers’ trades on their balance sheets”. It’s also down to the fact that lots of traders have left the business after the world – somewhat understandably – got upset and fed up with their constant manipulation of everything from Libor to foreign exchange rates.

You add this all up, and it makes the market less liquid. In other words, you can’t guarantee that buyers will always find sellers, and vice versa. That makes it more likely that big moves in price will happen, particularly if all the trades are being executed by trading programs, rather than human beings.

Indeed, just a few days before the pound slid, Bank of America warned of this very thing. Because trading volumes are in decline, reports Bloomberg, “transactions have a 60% greater impact on prices than just two years ago”.

Of course, none of this is helped by the fact that the pound is in no-man’s land. From a technical analysis point of view, there’s no “resistance” between here and the pound’s 1985 low – which was around $1.05.

I’m not a chartist, but put simply, it means that the pound would have hit a lot of price points at which people were keen to sell up (because of stop losses) and no obvious point at which they would have wanted to buy en masse.

You don’t start a negotiation with “OK, you win”

That’s the rough mechanics of the slide. As for the fundamental reason for the fall in sterling – that’s pretty clear. Markets are now worried that there will be a “hard” Brexit as opposed to a “soft” one, or no Brexit at all.

(It’s not all about the pound, by the way. The US dollar has also been strengthening as investors start to believe that the US Federal Reserve is serious about raising interest rates soon. That’s why gold had a crummy week too.)

The concern for the pound started when Theresa May said last weekend that Britain would trigger Article 50 no later than next March. Britain wants to trade. But it also wants control of immigration.

The immediate assumption of most people is that this means Britain is out on its ear. The laws of physics of the EU state that you can’t be a member of the Single Market and also opt out of freedom of movement. Therefore, there is no middle ground.

This, of course, is nonsense. The EU’s rules are not like gravity. They’re not even like the Ten Commandments. They are an agreement that can be altered. Britain is currently in the process of negotiating changes to that agreement.

And you don’t start a negotiation by saying: “We want A, but we’ll accept B. And if you bargain really hard, we’ll go all the way down to C. What’s that? You want C? Umm… fair enough.”

As May said in her speech: “History is littered with negotiations that failed when the interlocutors predicted the outcome in detail and in advance”.

The EU is doing the same thing. You just need to look at the language being used on that side of the Channel to see that. Here’s EU commission president Jean-Claude Juncker: “I hope entire chunks of European industry won’t engage in secret discussions, in dark rooms, curtains drawn, with British government envoys”. It almost makes negotiating over tariffs sound jolly good fun – like something out of a Cold War novel.

Anyway, the overall impact is that the market got a follow-on fit of Brexit jitters. And with the pound being in a bit of a trading vacuum, and everyone keen to “short” it anyway, it tanked.

What it all means for investors

What are the implications from an investor’s point of view? Well, chances are, if you’ve remained diversified across countries, as we’ve often recommended, you’ve been pleasantly surprised by what’s happened to your portfolio since the Brexit vote. Practically any foreign-currency-denominated asset you own will have gone up in sterling terms.

And even the UK market has done well. After all, nearly every country around the world wants a weak currency. Britain is one of the few that has so far achieved this.

Will it carry on? The tricky thing with all this is that there are so many moving parts. Bank of England boss Mark Carney might now be a bit more circumspect about weakening the pound any further. So maybe lower interest rates aren’t on the cards any more.

The US election will have some sort of effect too. And you never know where a crisis will erupt in Europe next.

We’re suddenly in an era where politics really matter. And that makes markets even harder to predict. The only solution is to diversify.

As Merryn Somerset Webb pointed out at the MoneyWeek Conference last week, “diversify” sounds like boring advice. It’s not.

It doesn’t mean “buy everything”. It means that you need to make sure that the success of your portfolio doesn’t hinge on one particular asset or investment scenario. I’ll be talking about it more later this week, but it’s probably the most important thing that you can do right now. Meanwhile, if you missed the conference, you can pick up your DVD copy here.