Hi people,
following clues in various answers and doing a bit of detective work (on youtube) it seems that you can get the individual cells for these batteries at that on-line store for the famous mythical tribe of female warriors and probably the world famous auction site.
They are known as "sub-C" cells, don't confuse them with the 4/5 sub-C as they are a smaller cell of the same rating. You will need 15 of them for the 18v battery, which comes in at £19.99. https://www.amazon.co.uk/gp/product/B06XKB4MX2/ref=ox_sc_act_title_3?smid=A1O371PGKL7UNO&psc=1
Then, if you take your battery case apart and replicate the layout with a bit of careful soldering, you can re-cell the battery.
A couple of words of caution: you need to re-use the little white box bit in the same part of the circuit (a heat sensor I think); make sure your solder joints are good - to prevent arc-ing; but be careful not to apply too much heat as this can damage the cells; and finally, be careful not to short out the cells as you build the pack (as the bloke on youtube did when re-engineering a pack https://www.youtube.com/watch?v=wn6ruw_39MM )
I'm waiting for my cells to arrive and will update with any hitches I encounter. I came to the conclusion that this option was easier than the reworking of a Ryobi battery as that has to be modified with the layout of the cells to give the correct polarity at the connection to the tool.
Hope that helps in this throw away world...
Oggy
Oggy-ogg
April 2021