85k and I think my engine might be dying.

Expansion tank has developed a small crack today, this is either because I forgot to bleed the system the last time I filled it up, or I have been suspecting a head gasket failure for a while.

No mayo on oil filler, no oil in water, heaters working fine, actually red hot.

The crack in the tank is not huge but it is bubbling and dripping when it's been out for a run and warmed right up, it looks old, could just be through heating and cooling? Sometimes the cap hisses after a thrashing, but now obviously it will escape from the crack. It's holding water fine now it's cooled down, but as the temp raises I know it will do the same again.

I've got a bit of lumpy start on occasion, but not every time, and I had an issue with cheap HT leads shorting out, which I believed caused problems with the coil packs, which is why I bought new ones. There is a lack of power sometimes but not all the time. It's just not running smoothly in general.
I've got ignition coils to put on but haven't had chance this week, I've just borrowed money to replace all dampers and springs, which I was going to fit on Saturday at a friends but I don't know if I dare drive it anywhere.

Could do with new belts, rocker cover gasket and I don't know if the head gasket is ok. I took it to a garage to get a compression test and he said he'd do it if I really wanted but he reckons it's not the HG causing the issues, I reckon it's electrical but I'm far from an expert. I can hear a clicking noise, it sounds like shorting and it comes from the dash where the speedo is as far as I can tell while driving, as this is the only time it happens, and only over 20mph.

Also a few weeks ago for about a week, when I put the indicators on the usual click was followed by what I can only describe as a squeak like a mouse... This has stopped now.

Last service wasn't too long ago, new plugs, leads, oil, air filter, oil filter, coolant flush and oil flushes. After the service it was running nice but has slowly got poorly.
Didn't do fuel filter, and also when I fill the tank right up it leaks but only a drip and only when full to the top.

What am I dealing with here? Am I thinking the worst or is this (please be) various small issues? I can't afford to get a new car, I've spent quite a bit on this car and I've become quite attached to it so would like to save it ideally, but is it worth the money? New engine? Fix the current one?

I'd only end up buying another Seicento anyway....
Sorry the post is massive, just trying to be as detailed as possible. Thanks for reading.

I'd get a new expansion tank -- without it, the cooling system is not going to work properly -- it needs the pressure. You'll then be in a better position to monitor coolant loss.

If coolant loss, or signs of brewing up after you've replaced the tank, do a compression test. Cheapo testers on eBay, and it's a tool for life.

yep agreed. Change the tank, refill and bleed system.

Buy a compression tester, like £15 and so easy to do the test yourself - no need to pay someone!

edit: get a new cap for the tank as well if it doesn't come with one.

Got the ignition coils changed yesterday, expansion tank changed this morning, about to change dampers and Springs this afternoon and going to do a compression test after, to give it chance to cool down. What kind of readings should I be getting if it's healthy?

the crucial thing is they should all be the same, well very similar. Do a dry and wet test.

So, disconnect crank sensor so it doesn't inject fuel or spark, remove all the plugs and screw in tester at one end, hold the throttle wide open and turn the engine over for few seconds, note down the pressure recorded - repeat on other cylinders.
For the wet test put a few ml of oil down (just a little bit! like a teaspoon worth) down each cylinder before you test.

post results.