Please don't tell me it's the head gasket

[Bowline5]

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?