I've found a weird sediment in the expansion tank. It's not like oil-in-water mayo as it sits at the bottom of the tank and doesn't smell of oil, but is a bit worrying. The expansion cap is clean, as is the oil filler cap and it doesn't use any fluids really considering the abuse I put it though.

I've had to change the coolant a couple of times recently thanks to the abysmal design of the heater matrix. This has been in & out 4 times since Feb to try and fix the slight weep from the o-rings. I wonder if that's disturbed any casting residue.

Any ideas?

How long have you had the car? Is it possible a previous owner also suffered from coolant loss and used a leak sealer to try to fix it. Of the rad seals on the market, Barrs Leaks especially is quite granular and can produce quite a lot of deposit in the expansion tank.
I would remove and clean the tank thoroughly, you might have to buy a bottle brush to get to the bottom, flush the whole cooling system, change the coolant and hopefully the sediment will not come back. Good luck