Hello Pigginheck and thanks for posting, 

I can appreciatte you wanting more specifics about what constitutes heavy lifting, but I am afraid I have struggled to find a definite answer for you, this sort of thing may vary a bit depending on the circumstances. It would probably be best to try and pin your doctors down and ask them what they have in mind regarding actual weights, but it may be that anything more than a few kilos isn't allowed.  Doctors sometimes say a couple of bag of groceries is ok, but this isn't very helpful as the weight of these can vary quite a bit with the contents.  

The weight lifting limits for every day work purposes are given by the Health and Safety Executive. You can download their guide on manual lifting at work here.   If you look at figure 1 and the explanatory information in the guide you will see that even without any medical restrictions, lifting a weight of 20 kg is off limits for women, so I don't think you should have been lifting that much to begin with. This guide mentions that the awkwardness of a lift and the position of the arms has a bearing on the maximum weight.

I do think you need to be cautious and I certainly don't think you can do what you used to, so you will need to adapt, but try and get more input from your doctors to be sure. 

Wishing you all the best, 

Julia