I posted my h/u Royal Mail special delivery (from the post office) which arrived next day. The cost is about £7, but the item is insured up to £500 I believe.
The guy at GTS said it depends who you speak to whether they will accept the repair or not, as mentioned, they are downsizing, so have lost the contract with the courier, which means some of the engineers might not be willing to deal with all the trouble of return postage.
If you speak to them on the phone and have problems with them accepting the repair, the best bet is to:
1) Say that a friend got the service, can they check with a colleague,
2) Bribe them by offering £100 (they are a small company),
3) Enclose return postage packaging to make it easy for them to return the unit to you once repaired.
I'm happy to support these small independent guys. Lord knows the Stealers could do with some competition.
Hope this helps some people.