Many thanks for the replies to my original post. They do confirm what I suspected in that there is a variation in policy adopted by different operators. The Boeing 737NG FCOM contains the following paragraph:
Testing of undiluted de-icing/anti-icing fluids has shown that some of the fluid remains on the wing during takeoff rotation and initial climb. The residual fluid causes a temporary decrease in lift and increase in drag, however the effects are temporary. Takeoff operations with reduced thrust (assumed temperature method or fixed derate) are permitted provided takeoff performance accounts for the runway surface condition. Use the normal rotation rate.
Given the potential cost, in terms of engine wear, of not derating and Boeing's acquiescence with derating, I am very suprised that some operators continue to adopt a policy of not derating after de-icing.