Thank you very much for all your replies. I have not been able to upload photos of the sockets - but the new master socket is an NTE5 and the secondary socket is an older socket fitted by BT about 2004.
Thank you, Mr Saffron, for the interference suggestion. As far as I can see there are no electrical items nearby.
I have now also tested using the test socket, which unsurprisingly produces the best speed of all - 6673 kbps down and 578 up (router stats 8192/704). However the master socket still produces 4578 / 578 (router 5548 / 696) and the secondary socket produced 5529 / 552 (router stats 6692 / 689).
I suppose the figures point to a fault in my internal wiring - but I have tried disconnecting an extension line I thought might be at fault with no material difference. Would it be worth replacing the faceplate on the NTE5 in case that is at fault? On the other hand it would only produce about an extra 1 mbps so probably not worthwhile.