i have had a life insurance policy with L&G for 5 years. i am aged 40. can i add critical illness insurance to my existing policy to protect my mortgage?

No you cannot add critical illness cover to your existing policy. You could have bought a combined life/critical illness policy at the time you originally took the policy out, but it cannot be added once the policy is on risk. If you now want critical illness insurance, you will have to take out a totally new stand alone critical illness policy.