hmmmm. IIRC the threaded part should be replaceable without too much hassle, still thats a bridge to cross when you get to it. Don't take this at read, I can't remember for definite! Given the amout of effort you've already put in, I would suggest juduicious use (very very carefully - if you damage the mating surface on the block you really are f****d - and block the holes with something to stop bits of metal getting in the engine) of a mini grinder with a zip disc in it, to cut throught the mating surface of the filter at two points. this should release the load on the thread and allow you to undo it. Unless of course as you suggest some twonk has cross threaded it, but lets hope not.
Its a good suggestion clan, but the recess on the engine really makes getting any kind of tool into this awkward spot nigh on impossible. I'm considering fabricating something - unfortunaltely I don't have any welding kit, but if accurately measured a big block of wood, about filter size (e.g. saw a section from a fencing post I have kicking about) and drill a centre hole for the screw part and put six screws in for each of the oil holes. Then somehow fit a bolt onto the wood block to it and bring on my big ol' breaker bar. Failing that, I'll have to go buy a small grinder and try chris's method. The only I have grinder is big, and I really don't want to have to remove the pipework (which I think is power steering) to give me a really clear run at it.
I Wish it did, and it may do - I only have a haynes manual that covers petrols. if I could get the whole thing off the engine everything would become easy peasy as access is a nightmare. Its a renault lump I believe, but I've no info on it whatsoever... I wish I had a workshop manual or something for it so at least I could work out where the bolts are for it.. If I could get that part out of the engine that could be the answer.
Clan: Ummm.. hard to say. from memory there is some kind of block with two pipes coming out of it (one inch-ish in diameter) but I could not see any bolts. There is a lot of corrosion and build up of oil and grime and its really hard to see what's going on. I didn't look as to where the pipes were going to, I assumed they carry oil but I may well be wrong. I have took the rad out from desperation, I don't even know if I can re-prime the water system, but for the time being if I can't get this filter of I am completely screwed. I will get out tomorrow and see if I can make some bodge-up to get purchase on what is left of the filter. Failing that, if anyone knows the TD 1.9 engine well and knows if the filter mounting block with 2 pipes comes off, please get in touch and let me know how..