Thanks for all your help guys. In case anyone stumbles across this, I've added ~10kg by adding 2 layers of 'code 4' roofing lead to the entirety of the seat (attached using no-more-nails exterior), and then covering it with duck-tape to keep the adhesive free from water. This keeps it below the 'lip' of the insert.
The rest will need to be lead blocks which Club100 indicate they have for sale at the event.
First race is on Sunday so I'll be turning up early to make any last minute adjustments and fitting the green foam
