Welcome to the forum Dai.
That's tight!
I hope you don't have an Aristo Class 66?
Unless you are running Hartland short wagons, with a short wheelbase loco, it's just not going to work.
LGB R1 is 2 ft (600 mm) radius, so you will need a 4 ft diameter (1200 mm), and that is centre to centre, so realistically, you need 4ft 6 in of space.
I have a double track return loop (R1/R2) which just fits in to a 6 foot space, but I have to be careful as to what rolling stock can use it....