I have ended up buying some Westfield seats, but during my research I found a cheap supplier of Cobra seats:

http://www.sowdentyres.co.uk/colway-cobra-seats.html.

I spoke to the bloke a couple of times, and the prices (most recently confirmed on 4 August) were:

2x Cobra Roadsters, no harness holes, choice of contrasting piping (no extra charge): £295 incl. VAT and P&P

2x the same but with harness holes: £353 incl. VAT and P&P

2x Cobra Cubs, no headrests: £179.50 incl. VAT and P&P

2x the same but with "headrest facility" (i.e. metalwork inside the frame to support the headrest) and headrests: £258.60 incl. VAT and P&P

Seems good value compared with every other supplier I could find, including Cashy who has sold to a few people on here.

I can't vouch for them as I didn't actually buy, but it might be worth a try if you live near to them (Drighlington, near Bradford, according to the website?