Thanks. I spoke to TAC and it seems its a software bug so i've got to upgrade (heard that one before?).
The only issue is that the flash can't be squeezed in the 2600 so I have to flatten the whole router and tftp everything back on just to upgrade so will have to take that part of the network down for too long. I'm running a whole office through it, phones et al.
This is the problem with spaghetti coding devices.