does anyone know rules on chickens on allotment?

DEFRA needs to be informed for over 50. My old allotment lowered the number to 5 hens and no cockerels.

NG's right that on council plots there is a legal right to have rabbits or hens.

However, that is subject to an over-riding issue of whether the birds are creating a statutory nuisance such that environmental health need to get involved.  Usually this applies to domestic settings such as gardens, but I wonder whether the number / noise / smells / vermin attracted by the plot could constitute a nuisance in the same way.  It's probably a matter of fact and degree, but 60 + 20 birds does sound a lot.

If you needed to, you could contact your local EH officer at the council for advice.

There is also the question of what the tenant is doing with the birds, in so far as whether he's growing them (or their eggs) for onward sale - is this becoming a semi-commercial operation?  a) That's against allotment rules if it's a council plot (the purpose of an allotment is personal use only); and b) sale to the public involves a lot of additional regulation; it's not really covered by the 'farm gate' rules.  Again a question for EH, I think.

This may not answer your initial question but may give you some additional avenues to explore.