My 07 Forester has the EJ255 engine, 2.5l 16 valve turbo, on the way home a few days ago the temperature gauge shot up to the maximum. I limped the last few miles home, next morning the only obvious issue was a very low coolant level in the expansion chamber. I topped it up, everything is fine; no leaks, no clouds of steam or white smoke, engine smooth and pulling well BUT in a little less that 100 miles the expansion chamber level had gone from full to nearly empty, so where is the fluid going? The car had no service history when I bought it 3 years ago, it had barely moved in 3 years, it's done 173000 miles. If the gasket is failing it would be daft not to do timing belt, plugs, pulleys etc etc at the same time, US sites suggest parts and labour at $3000 to $4000 dollars. Does anyone have any idea what the UK cost might be?, or where there is a good aftermarket specialist who might look at it?

It highly likely head gasket the issue, check some basics such as water pump and thermostat housing/hoses for leaks and also radiator and turbo cooling lines (visible coloured staining marks left from antifreeze)
You can use chemical test kits detect combustion gases in cooling system (we use gas analyser over rad filler neck)
It done some miles and to be honest I wouldn't want do just gaskets at that mileage, bearing shells, all new seals, new oil pump , new rings/pistons and valve overhual tends be best way forward but you could indeed do just head gasket and basic head overhaul if the short block seems decent enough and assemble with new water pump and quality japan timing kit (ICP do these at sensible money)
Good parts and good effort won't be that cheap & at this mileage further issues could crop up from fatigued engine stressed from the poor cooling/overheating .

Thank you Mr B, that's a depressingly long list of things that might sensibly be looked at. For the time being I can manage it by keeping a careful eye on the expansion bottle, the car is running very smoothly with no loss of performance. Now I must think about what to do and where the money will come from. Much as I like the car I think it might have to go to that great big car park in the sky.

If you intend fixing it the quicker the better as longer left like this potentially more engine damage is caused .
I have done head gasket only (resurfaced heads) on forester turbo's before and engines lived on fine but partially only due to being pretty confident on engine health prior to doing and fact mileage was fairly low .
It be a big gamble at your high mileage doing just head gaskets should that be the issue .

If you do decide do head gaskets you want be very selective on parts used and quality of work .
The boxer engine is pretty easy to pull and do the heads for someone with experience with them .

Option 2 is check the basics and if nothing found assume it head gasket as that the common failure and just live with it for six months to a year while work out a replacement .
 

I've taken the underpan off and had a good look all around. With the engine running I discovered a very slow drip from behind the timing cover, it makes me think the water pump is the problem. A heavy duty leak stopper in the radiator seems to have stopped it for now, a breathing space while I source a timing belt kit and other bits and pieces needed to replace the pump.

Only kit to use is the ICP timing belt kit with OEM manufacturer sourced parts.
They also do GMB or Aisin water pumps that pretty good (use a oem metal gasket with it)
Cost effective and decent parts ...
ICP do sell on eBay or via website ... calling them is good option.