As said if it needs head gasket I would just have it replaced, but then it has potential other big problems that could bring the bill past another £1000 + and then if I do all the big jobs I may as well do all the small jobs which could add up another £300, however seeing as it has already had £2,000 + on repairs in under 2 years and the retail value in good condition is around £800 - £1,000, I'm wondering is it worth it ?
if thnis steral seal would work would be great

if it even is the ehad gasket, what car suffers apparent hg failure but 15 months later still drives without loss of power, no smoke at all (not even in cold weather), no oil/water mixing, just needs topping up once a month or so or fortnight