Even if they are £300 in parts, does it matter? It isn't going to need doing again soon and is it not worth replacing it all properly in case something that you don't fails a bit further down the line?
I need to do mine sometime too. Need to get a list of exactly what parts are needed though, is it just belt, tensioner, and idler? I wouldn't have thought the crank pulley needs doing.
As an aside, if anyone's going to do their gearbox oil, a) I'd recommend it and b) you don't need to fart around wedging the gearbox away from the subframe, just cut a little bit off of an 8mm Allen key-about 15mm, then it'll go in the plugs and you can use an 8mm spanner to loosen and tighten it. Pretty simple job.