Does it make a bubbling noise when you squeeze the top hose? (other than water flowing). If not, then it's refilled now. Only time will tell if the gasket had been overstressed, if it goes for 6 months+ without adding any more water it's OK. Mine was refilled once in the stealer car park (got warning as soon as engine started turning over), and again after 25miles, and started losing coolant noticeably after about 4 months..

I didn't hear any noises when I was checking it. I didn't have the top off the expansion tank though when I was doing it. Will try that.

Today I tried running the engine for about 10 minutes with the top of the expansion tank off. There started off being a few little bubbles in the expansion tank but they then disappeared quite quickly. Check the top radiator hose which felt squishy. Also before I started the engine, I checked the oil cap for any 'mayonnaise' which might indicate problems, but none was there. Going to keep an eye on it in the coming weeks but hopefully all is good.

I hope it is OK, though you won't see anything in coolant as initially it is exhaust going into coolant, pressurizing it and expelling through the bottle cap, only under full throttle, e.g. entering motorway. But if it's OK now, best not to worry ahead, give it at least a few months and then hopefully you can forget the experience.

Yeah I'll find another garage in future as well! Thanks for your help.

Coolant warning light came on again straightaway when I started the car this morning, checked the level and it was near the max mark! Restarted the car and the warning light came on after about a minute. Restarted the car again and it didn't come on. Took it back to the garage who did the cambelt and water pump and they didn't have any ideas, said bring it back if the warning light comes on again. Having done a search on here though I may need a new expansion tank as the sensor in that usually goes. Does that sound right?

Yes. The cost of a new expansion tank is under £15 and it doesn't take long to replace.