Have just bought a second hand Cyrus 3 amp, which has come without 3mm banana plugs for the loudspeakers. They are proving very difficult to locate (Cyrus and my local Cyrus dealer could not help). Any recommendations for where I could buy these plugs?

Do you mean 4mm?

http://www.maplin.co.uk/Module.aspx?ModuleNo=44115&criteria=banana%20plugs&doy=10m3

IF only they were 4mm! Unfortunately Cyrus used 3mm plugs for a short period.

You could use Z plugs. If you push them in to the sockets, they'll collapse in on themselves to fit.
http://www.hificables.co.uk/ProductDetails/mcs/productID/10959/groupID/710/categoryID/2397/v/877a7abc-6e74-4145-9f55-8b68d981173d

Otherwise, use spades.