A lot of people, myself included, have fallen foul of this little issue when fitting the Goodridge braided hose kits. Basically, they need to be 12mm, like the fronts, but Goodridge supply 10mm fittings in their Vectra-C kits.
Seeing as I'm fitting new brakes to my Signum, it seemed silly to know get a set of the hoses, so I contacted Goodridge directly to see what they'd say about it and what they could do.
I recieved a reply from Goodridge this morning:
Good Morning Martin,
Thank you for your email. I have had a look at our kit listing and all our banjo's in that kit come with 12 mm heads. You shouldn't have a problem with fitting that kit on your vehicle.
Our largest stockist is a company called C.A.M Auto Developments, Contact Ian Goldie 01727 827717. He should be able to help you out with queries and supplying you with the kit.
Now, we know the above is a load of rubbish, so I looked on the website of the company they listed and found the Vec C kit. It doesn't mention specifically the sizes, but it does say they wanted £90 for the kit, which is a good 40 squid more than everywhere else...
So, I did a bit of digging, and contacted http://www.brakes4u.co.uk to see what they could do. Spoke to a very nice chap, who listened to the problem and vaguely remembered having something similar to it before. He took my order, and included a note for the 12mm rear banjos as they order the kits in only when ordered. Full range of colours and the banjo material spec.
Total cost was £52 as i ordered a different colour which was £3.50 extra, but that includes free shipping. So if you order the plain bog standard ones, it's less than 50 quid...
Contact details are here: http://www.brakes4u.co.uk/infopage.asp?id=1