Are you absolutely 100% totally positively sure that it isn't gearbox fluid?

Sorry to hear this but it is probably nothing to worry about.

I assume the garage checked the ECU fpr fault codes and then cleared them.

What state is the coolant?

There are untold millions of ECU’s being driven around in every part of the world doing, without question, what they were designed to do.
From the moment of their first boot until they are decommissioned (in whatever route that might take) these obediently perform with unerring accuracy their tasks & although there are glitches in systems these tend to be external influences; moisture tops the list.
“Experts” who blame problems on ECU’s are more interested in your money than the health of your car.
Whatever new witchcraft motor manufacturer institute in their new cars, will be the major contributory factors to your motors ill health until the next brainwave is foisted on to us, irrespective of the true problem.
Or am I just getting disillusioned with this motoring caper & the folk who profit from it all.
M

ECU calibration issue?????? unless your ECU has been remapped and badly at that.....then I suspect they don't have a clue what they're talking about perhaps they work to the principle of:
If you have no idea of what you're doing, talk a load of shit, with plenty of technobabble, gravitas, authority and a straight face, in the hope you bamboozle the audience.
Politicians TV presenters and senior management do it all the time!

My little box of left hand cobalt drills have paid dividends (thanks for the tip Cobb’s). The heat created & the “reverse” direction frightens the little buggers right out of their rust pit. You need a reversible drill obviously, most of us will have battery drills as normal.
I have used Allen headed stainless screws with a dob of copper grease on all I work on but do run an M6 tap down the hub hole to clear out all the rubbish.
You will most probably do this as a matter of course but I included it as an aid memoir for those who might be new to greasy hands. Copper grease on the cleaned up mating surface between hub & disc.
M

I've always found the only way to remove disc and drum brake retaining screws it to use an impact screwdriver

give it a good whack with a 4lb (2 kg) hammer

and they should then undo easy

If you like to use Haynes manuals you can use a combination of the Rover 200/25/ZR for the engine and gearbox and the Austin Metro/Rover 100 for a guide to most of the rest of the car.

Actually I'd definatly give the car a service and do the timing belt, but other than checking for problems I'd leave the head gasket alone......Don't mess with success!
Compression tests, cylinder leak down tests, coolant sniff tests and coolant system pressure tests will indicate head gasket problems as will an inspection of the oil quality, snotty oil will not do! and is an indicator of head gasket problems. But it these all pass then leave the head gasket alone.
Don't rely on service records as the work may not have been done properly.