Vauxhall Corsa problems point to head gaskett leak but garage find tests come up fine

I think you need to change your garage. It sounds like they are ripping you off. Go back to the garage that said it might be the head gasket and get them to take a look....

See if you can find a garage that can perform a coolant dye test.
In this test if there is ANY combustion gas present in the coolant,it turns the fluid in the tester from it's straight from the bottle dark blue to either green or yellow.
I usually charge £20 for this test,recoverable if the gasket is gone and I get the job,I set the tester in the expansion tank,engine hot and running at idle speed then I go get a coffee and let the customer watch the dye.
by the time I'm back either the fluid has changed or not,you can see,you can decide,if there is no colour change then the gasket is sound,definative.
I want your money,but only by repairing the fault.

before checking for head gasket they should be doing a pressure test on cooling system to check for external leaks (basics first)if no leaks are found then do a head gasket test you say the water is empty again,how quick does it lose its water also i would do as others have advised as sounds the the fitters at the garage have no clue what they are doing

I would take your car to a garage that is recommended to you by others who are satisfied with their work as these people seeem to be more interested in your money than the solution. As made in previous comments the first thing would be a Gas/Block test, most garages would have this and the now loss of power would proberly also suggest this. Unless obvious any overheating I would a;ways check for the head first as would be to eliminate the most expensive part of a job before hand. If they were a good garage they would fix the problem with a discount as they have had a fair chunk of money already. Anyway Proberly is the Head gasket now. Find a reptable garage, don't go to ones who's staff work on commission. And make sure they give you the full cost before carrying out the work and if it will be guaranteed after for a short time for piece of mind. Good Luck