Vauxhall Corsa losing water from expansion tank

My wife’s runabout overheated a few weeks ago. Checked the water, which was low, so topped it up and it was ok for a few days. I kept checking the water level and had to top it up a few times. Each time, I checked the oil for contamination, but oil seemed fine.

Now, last fews days the engine’s been sounding ‘tappety’ and yesterday it overheated again – this time the oil had turned creamy, so I’m guessing it’s the head gasket?

Took it to a garage yesterday, but they can’t fit me in for another 12 days. I’m just trying to prepare myself for the phone call with the estimate for the work! Any ideas?

The car’s probably only worth £850 tops, so what would people suggest is worth forking out to get it fixed? Would it be prone to failing again once it’s happened the first time?