Hi, insurance is a real minefield, and there's been some quite good posts about this in the past (am at work so won't have time to search for them, sorry hun).
The most important thing to check is whether the amount paid for a condition is a per year sum, or just up to a certain amount and then it stops. Its best to get one which covers a condition per year, as if its onging, they will carry out paying, whereas if the cover is only up to say £5k, once that runs out, that's it.
M&S were always very good, a few of us have gone directly to AXA (as they were the underwriters anyway) and saved a bit of money, but I think they've increased their prices to match now.
Defo worth shopping around to get the best deal. Personally I've had no dealings with the companies you've listed, so can't really comment on how good/bad they are.