In reply to Jimbo W:
Yes, this is almost certainly the cassette you want - 28 tooth.
> 2) The rear derailer should be able to cope with all of these cogs? And if not, which bit is it likely to struggle with? The largest cog in the rear cassette?
It's not ideal but probably will work good enough. You are likely to be unable to used the big sprockets in the big ring but this shouldn't be too much of a loss.
> 3) On the front chain set, I could go down to a 39x28 (36"). Do I need to source this from Campag?
Doesn't need to be Campag, you should be able to find a cheaper compatible one. (3/32 - 9-speed) you need to know the bolt pitch (BCD), yours looks like BCD 144mm but I'm not 100% sure could be 135mm I suppose.
> 4) Will I need a new change for either or both of these changes?
Changing the front ring to 39 will not require mech changes. 28 rear might.
> 5) It looks like the front derailer is a Campag Mirage. Am I likely to need to change this?
Not if you go with 52/39, if you change to the triple chainset you might.
> What's the best place to source this kit?
Pick from Ribble, Wiggle, Chainreaction, Parker International, Rose bikes.
> Is the gearing quite tough as it is? I mean, it didn't feel the easiest going up the hill for me, but it was doable.
Let's just say that gearing range "isn't fashionable".
The idea of losing the outer ring from that cheap triple is likely to prove unpractical due to how the front mech attatches to the frame, the need to chop or replace the chainring bolts and posible axle length compatiblity issues. To overcome these issues could be technically difficult and/or expensive.