That does seem high, Truthmonkey, but ours was about £35 and the woman in the launderette said it was the drying - I think £1 makes it work for something stupid like five minutes - Abbey Street launderette. It was still damp too.
The launderette on Long Lane/Crosby Row has large machines
which are ideal for duvets and they do service wash too.
I have had mine done here before if I remember right last time it was about £9 as a service wash. They part dry in the machines and hang them out in the shop at night to reduce the cost. If it cost me £40.00 to clean them I would just buy a cheaper duvet and replace more frequently.
I agree with jackie, take it to the laundromat and if you have the space hang it over a door..I would always be dubious about chemically cleaning duvets as several years ago a married couple died after taking their newly cleaned duvet out of the plastic and placing it on their bed instead of airing it completely.