New read mech for my dawes shadow?

[dasy2k1]

I have an old 80s Dawes shadow with what I believe to be the original SIMPLEX rear mech which is understandably practically worn out

It has new wheels (3 years old cheapo alloys) after some scrotes kicked in my old steel wheels presumably when they discovered that my lock was not going to snap with a simple twist!
Rear sprockets are a standard freewheel with a 6 speed block. 14/28t range
indexing is not a problem as I have friction downtube shifters.

can anyone recommend a new mech to go with this setup?

 

[dasy2k1]

Have degresed it and re oiled it up for now and its running a bit smoother, good for another couple of hundred miles, but will need replacing sooner rather than later