Try a google for TowSure and see what they have.
I looked into this a few years ago but was never able to get another van so never followed it through. IMHO the type of rack that fits on the spare wheel carrier will cause problems. firstly your putting extra weight on it and also putting movement to it. The T2 spare wheel bracket tends to break it's spot welds and starts to creak and squeak, don't worry it wont fall off as it's bolted too but the extra weight and movement surely wont do it much good.
Best type that I saw have a bracket that fits on the towbar between the ball and bar. The carrier fits in to the bracket with or without a caravan being attached. All the weight is being transmitted to the chassis and you've got something solid to chain the bikes to.
Jim