I had critical illness cover on my mortgage even though we switched over to a repayment mortgage we still kept up the endownment "just in case" and I am glad we did. I didn't even think to claim it until someone mentioned it to my mum and then I spoke to my Consultant's Secretary and my bank (then Abbey National) and the ball started rolling.
They sent a form to my Consultant who filed it in and sent it back and I think I was paid out within about 5 to 6 weeks, it was enough to pay my mortgage off and keep some in the bank which we have "forgotten about".
I hope this helps
I have posted a link to some threads where critical illness cover has been discussed in the last few months, I am sure your fellow users will be along soon with their experiences too: