Is the 1 1/8 fork more common than the 1", sorry if its a daft question but im looking into up grading my forks to suspension and i was told my bike has the 1" headset/fork, if so is it possible to upgrade to the 1 1/8 ??
you can't use 1 1/8 with a 1 inch steerer is it simply too big, 1 1/8 is more common in suspension forks but a lot of retro forks came in both sizes and some have steerers that can be swapped.
If your bike an older frame it might not be designed for suspension, if that is the case I would not put suspension with more than 65mm travel on it as it will upset the handling.
Chris King makes special headsets that will allow you to use a 1" fork in a 1 1/8" frame.... Some other companies sell spacers to allow you to use the 1" headset and fork in the 1 1/8 frame.... Both work great, but the Chris King is the coolest... I believe it is called a "Devolver"
EDIT: missed the part that said he had a 1" on the bike...
Suntour make a 1" sus fork, nrx7000 about £50. Recently fitted 1 to a 96 raleigh genesis, 63mm travel so not to mess up your geometry and works a treat around my local slate quarry's..