I have an RT3,and ive replaced the original carb with a Mikuni 32mm,as opposed to the standard Mikuni 32sh.{with flange}. I bought a carb inlet rubber to fit the new carb to the original alloy inlet with reeds.Thing is it doesnt fit,do anybody out there know where i could source one here in the UK

Is it far out?. On one of mine i slotted the holes to get them there & used allen screws as the heads are round & could get the allen key to operate where nuts would be difficult.

Mikuni might make an adapter if that's what you need?  They probably won't have it listed by model for a bike that's over 50 years old so you'll need dimensions for the bolt spacing and spigot size.

Here's a list with dimensions and Mikuni part numbers...  one thing is Yamaha often specified smaller body carbs than the "generic" carbs you buy today.  So the original 32 might have had the same body dimensions as a generic VM28 does today.  prelive.nichecycle.com/carb-flange-carbu...ubber-by-mikuni.html

(I know the original Yamaha-spec VM28 and even some of the original VM30's back then had the same body size as a VM24 today...  unless you find NOS, a new off the shelf VM28 will not replace the original because the body is too big compared to original)

Best bet is almost always to find an original carb.

The holes in the rubber boot dont alighn with the threaded studs,and when i fit the nuts there is not enough room for them,as if the circumfrance of the nuts is too big.This then distorts the boot,really need pictures to show this.I thought this was a common modification,to put a later model carb on ,as the originals are hard to find

It was very common.  First you need to get the correct adapter.  Second you need to remove the studs and use Allen head bolts.