Going to assume you have the same note acceptor as mine, which is an NV8..... Unfortunately, these noty's can NOT be recalibrated for the new polymer / plastic notes, so you will need to replace it with a more modern one..... NV10 USB (Has to be the USB version, as the standard one will not work), or anything newer than this in the NV series of mechs, should work, however it's not cheap.
Some info here for Thomas Change Machines, which will give you an idea of the issue, and what they recommend their users to do (Also contains info on coin mech updates too)....
Beware some suggestions will not work for all machines, especially the CF126 to replace the MS / ME range of mechs in some MPU4 based machines (Due to some voltage issue from what I can work out).
My manual says it has a NV10? I haven't checked the actual note reader yet though, if I do have to purchase a new one to accept the new £5//£10 is it possible to set it up so it pays out with notes as well as coins?
Thanks