Thanks everyone for the replies. I had a look at the packet of MG compost but it didn't specify the balance of the fertilisers. It does say that it contains enough food to last the plant for three months though so there should be something in it! Sadly I live several thousand miles away from Oregon, otherwise I'd give that Sunny Girl Gardens a shot.
Anyway, so I've now repotted in 15cm pots with a mixture of compost and perlite (I threw the original soil away) and from now on I'm going to use bottled water with a pH of 6.5 to water them (Clearview Spring, 25p/2L from Tesco if anyone is interested), so hopefully this will solve the pH problem. Our tap water here is very hard in addition to being quite alkaline, so that probably didn't help things much. Here's a couple shots of them in their new home.
I'll post again when I've done the pH test on the old soil and after they've had time to get over the shock of transplanting.
Thanks again!