A mate's just asked me the best place to buy a media centre PC. I know nuffink about this technology, and if I did would attempt to do it out of a mac mini anyway, so wondered if you guys would know?

I think most of us would probably build one. I wouldn't have a clue where to buy one!

Is there a budget?

No, but yes! It's a mates ex-girlfriend. There's no particular budget, but when she finds out how much it's gonna cost, I expect the interest will end there!

I'd get the folks here to tell you the best bits to put inside then give scan and CCL a shout and get them to quote to build it to your spec.

Not a bad plan.

Compro S350 for freesat

Well you could probably do something reasonable for £400-£500, but no doubt Sas will be along shortly to tell me that's nonsense.

I would go AMD:
Phenom II X2 550 BE
Asus M4A785TD-V EVO
2gig of whatever you wanna put in (or 4 if you can stretch)
Corsair 400CX PSU
for just video and audio over HDMI, a 4350 will do. For gaming, you'll need something meatier - perhaps a 5770.
Samsung F3 1TB HDD

Not sure on chassis. There are a million available and I don't mind a full size box, so I know nothing about the little ones.