Sorry if i have missed it Alex, but how big a spacer is needed? 
I've had to use a 5mm spacer either side. Currently they are non spigotted and the wheel doesn't locate on the hub and I get an 80 mph vibration. I picked up some 5mm spigotted spacers this morning, but I will have to machine down the end of the hub slightly to get them on fully.
If I didnt have the TT hubs, arms etc then I would just put a 10mm spigotted spacer on.