I'm with Direct Line. Granted that Lexy was pretty healthy with no pre-existing conditions and got that for £8.50 a month with a £55 excess per year.
Lexy had an injury to her back knees and had to have surgery, after care, all the tests etc, cost nearly £3000 in total in 2 months, and I only paid out the £55. All the vet fees etc were covered. I have lifelong cover for her for this and I get up to £6000 per year per condition.
As Jess has a pre-existing condition you may find they won't cover him for this, I am not sure there are any that do cover any condition pre-existing or not.
Also the £8.50 is for the advanced cover which includes loads of extras in case I'm in an accident and can't care for her etc and other vet fees as well.
M&S I heard are very good too, as Milly's Mum said.
I have found though that Direct Line don't ask many questions and they pay up fairly quickly if you have to outlay any cash up front like vet fees.
This web site is good:
http://www.aboutyourmoney.co.uk/pet-insurance/cat-insurance.htm it also gives you an indication what the maximum age of the cat should be that they will cover. As you can see Direct Line is first but if you cat is older they won't cover. There are some with no age limit.
Read the conditions really well and understand what exactly is covered before you signup.