I had that sinking feeling when a letter arrived this morning offering to renew our annual travel insurance, subject to repeat medical screening for Val. However, I need not have got anxious because it all went fine with medical screening and the policy has been renewed.
In case it is of help to anyone else, it is with Age Concern (you do not have to be aged!); they adopt an interesting approach to medical conditions which is to use their standard premiums but to increase the excess, in our case to £250, just for any medical claims related to CML. Our joint annual multi-trip policy for worldwide cover including USA and Canada is at a cost of £188 (ie £94 each in effect). The medical screening was sympathetic. They are, unsurprisingly, less concerned than some other insurers about covering you as you get older.
Age Concern's telephone number for travel insurance is 0845 603 4526 and their web site is at http://www.ageconcern.org.uk/AgeConcern/travel_insurance.asp - you can see the standard premiums there.
David