Speed limits

I have searched, without success, for more detail on UK speed limits. We have a motorhome over 3050kg so from the governmnet website I can see that we can do 30 in a 30, 50mph on single carriageways with national speed limit, 60mph on a dual carraigeway and 70 on motorways. My issue is on single carraige ways with 40 and 50mph limits and dual carriage ways with reduced speeds, there is one near us where the limit is reduced to 50mph. Also on the A17 there are several areas where the national speed limit on this single carriage way is replaced with a 60mph sign, What speed should we do here? Please could someone produce a comprehensive list for all classes of vehicles and road spped limits.

I have tried the government but had no reply. They are a lot quicker when they want tax out of us.

Colin