Seal belt?

For the record I'm in UK.
Peugeot Partner Combi Escapade MPV the twin rear seat is in folded position and I cannot raise the back of the seat, the belt seems to have gone in to 'lock mode' A mechanic friend who works for Merc says it's a common fault the manufacturers don't tell you about its the 'Jolt fault' when folded the belt lock in 'arrest' state so you can't raise the back of the seat. Anyone had same problem ?
What was yours or garages findings /solution?
Many thanks for taking time to read and or reply.