Dan, id say replace the plugs and leads and also bleed the cooling system then take it from there. you will be able to plugs and leads for the price Lee has put all in from a motorfactors
one step at a time mate, if yu cure the issues on the cheap then excellent, if not its service items anyway mate so it will help long term anyway
My garage did two for free for my when I suspected HG failure, they also did it again because they mechanic didn't have anything else on basically and thought he would check it! LOL
If you take it to a reputable garage and say you suspect HG failure, they may do it for free for you.
Ouch, get your water pump checked as GF had same prob on punto and that was a failed core plug that then stopped water pumping round that then lead to HG failure. I'd ring round only £200 for that fixing!