Hopeless how? Wow—I was completely expecting to see "product no longer available" looking for that 20 year old part, but the US parts catalog still shows both the left and the right belts here, for $346 suggested list price. But surely you will need to look in the UK parts catalog.
I know of at least one business (in the US) that says they will install new webbing on seat belts for US$75. You take the assembly out of the car and ship it to them; they rework it and mail it back. I don't know if they accept international orders, though. There may be a business somewhere in the UK that offers the same service. (Someone I know had the US company replace the tensioner on a Honda seat belt; that was $65, vs about $400 for a new seat belt from Honda.)
I presume this is the driver side (US left side) seatbelt. If you are interested, contact me by private message; I may be able to help. I'm in the USA. Just looked at the OP location: Edinburgh, Scotland? Any PriusChat member in the UK able to assist?
