Rebuild the Carburetors
I got a video tape on SU carburetor rebuilding, a Haynes manual and a copy of the Buren Fuel Systems rebuild manual. Buren is the current licensee of the SU products in the UK. The Buren Workshop manual this the best tool you will be able to find for doing the job. The video is good for confidence building but you need one of the hardcopy manuals to get through the step-by-step for a quality job.
The most likely problem with your current SU carbs is the wear that takes place on the throttle shaft bushings. These bronze bushes take a lot of abuse over the years and the bushes become egg shaped from the spring tension of the linkage and allow air to pass behind the butterfly valve. More air means higher RPMs. The amount of air leakage tends to vary at each position of the shaft and when you let your foot off the gas the air leak is never the same. This causes erratic idle speed and in severe cases will give you a high idle you can't tune away.
There are two choices for restoring the throttle shaft bushings. If they are not too bad, you can purchase .010 - .015 oversize shafts and ream out the old bushes to match the new diameter. Moss carries standard and oversize throttle shafts. This worked well for me. I did have to buy a $57 in-line reamer from McMaster-Carr (www.mcmaster.com). The reamer is adjustable across a small range so you have to buy one targeted for the size hole you have and it has a shaft extension that attaches on the end. The extension slides into the bushing hole on the other side of the carburetor body. this forces the reamed hole to be exactly lined up with the one on the other side.
The other method is to ream out the old bushings completely and the replace them with standard size new bushes. If I had to go this path I would have sent the carburetor body out to one of the rebuild services like Apple Hydraulics or Joe Cuerto.
Once you have decided on how the throttle shaft bush problem gets solved the rest of the job is well coved in the Haynes or Buren manual. You can get "master rebuild" kits from TRF, Moss or UK sources. Most folks recommended that change the float valves to Grose Jet brand replacements valves. The TRF kits come with Grose Jets included and no shafts. Follow the instructions very carefully concerning the correct alignment of the butterfly valves and centering of the jets and all will go well..
The job is well worth the effort. I have a rock solid, steady idle. It also gave me a chance to clean up every thing so the carbs now matches the new engine look of the rest of the engine bay.