Hi guys .I have received one via a car breaker on eBay. To my surprise he was happy to only sell me the cover I needed and not a complete part.Only £4.
My local Toyota dealers were able to come back to me after contacting Toyota uk directly.
So for future reference.
A pillar alarm sensor
Part number : GBNGA-VCAUS-BL £24.21 +VAT trade
Might help someone one day. I bet I find it under the seat as well now lol