jxk
There is two ways in which the belts could be reupholstered legally for an EASA aircraft.
1. They could be re-webbed by an appropriately approved Part 145 facility, using a C (component overhaul) rating. This is providing that they the approved data to do the work, i.e. the manufacturers repair /overhaul manual. Note: Pacific Scientific have such a manual, but it is approved only for use by their service centres, no others.
2. The work is carried out to an EASA approved minor change (modification) by a Part 145 using the fabrication of parts route or the C rating route. The change approval reference must be quoted and if the belt has an original TSO tag installed, the TSO approval must be scrubbed through as this is invalidated when another organisations change is made to it.
I can personally vouch for Pacific Scientific, having been involved with there UK and German operations in the past.
Hope this helps
ACX