Rik,
Give Used Tackle a phone on 0115 9631683. they used to sell the panasonic pin type battery for a loafer type float which used an led in the tip, the battery is a slim pin which is reversed in the float for storage.
They do mail oder or if you're close enough the address is:
28 Baker st
Hucknall
Nottingham
NG15 7AS
HTH