I have a pair of 40 DCOE that have been stood for 20 yrs.
Can anyone recommend someone to service them at a reasonable cost ?

Dave

Ive used The Carburretor Exchange in Leighton Buzzard several times over the years , worth a phone call as the advice given has always been good

DIY? I did.
full service manual here
http://www.lotuselan.net/uploads/weber.pdf

I had a set that had stood for a similar time. I dismantled and cleaned them and they were good to use.
They are made to be taken apart for altering jets. I do not think you need a to pay someone to do the work.
There is a book on DCOE carburettors, which you can probably find on Ebay or Amazon.
Should you need parts a good place to get jets and service kits is:
www.fastroadcars.co.uk/
The man is knowledgeable and helpful.