Jane,
I've been looking around at quotes and found the following that are worth looking at so far...
The AFI pet insurance link you've been given below looks quite good. More Than however not only match them but are slightly better and I got a quote at just over £10 a month (much less than half the cost!)
Sainsburys bank came in at between £15 & £18 a month depending on excess and maximum fee per condition (2500 for £15 or 6500 for £18). They have a very good policy and a lot better than both of those above and with no age or time limits and they've won awards for it in both of the last 2 years.
The AA (cars not alcohol!) also have a very good one and comparable with sainsburys. They have an extra option though. You can go for the standard excess where you pay a set amount for any treatment, or you can have your premiums reduced if you pay the first 10% of any treatment.
Personally I think you should look at the sites below first. They tell you all about pet insurance, what to look out for, what's important and all that and they compare all the sites for you. Dog people know dogs but insurance people know insurance!
Pet Insurance - Compare Cheapest UK Cat & Dog Insurance Quotes Online
Low cost Pet Insurance Comparison Guide - InsureYourPet.co.uk
Top 10 Pet Insurance Quotes Online UK - Compare Best Quote
Pet insurance | cheap dog and cat insurance quotes - moneysupermarket UK