Hi Patrick,
well done on the MOT pass and interesting that you have a ball joint on the way out.
Marlin are currently supplying 'First Line' which is what my local motor factors tend to sell.
http://www.flickr.com/photos/5806804...57626800104389 (Ian Morris may have the part number?)
But I have no idea if they are any good.
I would avoid buying anything that is not in a recognised packaging. I have collected a couple of new old stock 'Unipart' original parts as spares that do come up on eBay from time to time.
I think there are two different types of metro ball joints and I think the ones you want are GSJ269 but to be honest I have not yet fitted any of these.
Out of interest, how many miles have you clocked up now?