Hi, Im not sure what work they carried out but that seems a lot of money!
The idle smoothness of any engine is dependant on quite a few factors so unless you're certain other adjustments such as tappets, carb balance etc are correct the best thing would be to check those first. There can also be air leaks or wear in the carburettors themselves which will also make it very difficult to get a smooth tick over however the good news is that the adjustment itself is very easy.
Our cars as standard are fitted with very simple carbs where the accelerator pedal just operates a butterfly valve in each carb to increase the amount of fuel/air mix going to the engine. The tickover setting is purely achieved by holding these butterfly valves open a little when the pedal has been released and this is done by a screw on each carb butterfly operating mechanism. They should be easy to see if you locate the accelerator cable (which must be slightly slack) and need to be undone a little and by the same amount on each carb while the engine is running and hot.
If you cant maintain a stable tick over at about 900rpm then you will need to look at some of the other factors that I mentioned earlier....