[CR]Worn cotter pins?

Hi guys, please for give the lack of proper technical language, but I have a technical question about an on-topic bike. I am currently riding around on 1957 Allin Stan Butler Special (more detail and pictures can be found here: http://www.mrtwig.pwp.blueyonder.co.uk/SBSpecial/ ) which I am slowly but surely fixing up mechanically (it's a lot more ridable than in those pictures, but visibly not much different) while keeping it looking old and rusty as a theft deterrent and because I cannot afford a full restoration. Mechanically I've had no real problems that a little oil wouldn't cure, but recently I noticed a pronounced 'waggle' to my pedals. I'll try to explain that. If I hold one pedal firmly in place so it cannot move I am still able to waggle the other pedal a good 1/8th of an inch backwards and forwards. Something somewhere is loose or worn by years of riding. Now my hope is that it may be the cotter pins, a they would be cheap and easy to replace, compared some alternatives. Can anyone offer any suggestion as to the most likely cause/ fix before I start disassembling my first BB, as I have no other bike, and really need to keep this one on the road, so I would like to make this fix as quick and painless as possible.

Many thangs in advance,
Iain Grant, London, England.