Is it acceptable to sleeve the fuel filler hose with a smaller diameter hose in order to successfully reduce the internal diameter. I'm thinking of slipping some over the 1 1/2" tank neck then clamping some 2" over the top with an appropriate jubilee type clip. Only thing that bothers me is that 5mm 'overlap shelf' that will be on the inside

As the inner tube is flexible and the jubilee clip will tend to crush it, I think you may be in a grey area.

Why not make up a reducing tube out of exhaust pipe or similar, no step to worry about either.

I've only got 5" between bottom of filler and top of the fuel tank so the commercially available exhaust reducers would be too long as they're stepped in 3 stages. Could I use steel tube? I'm thinking about corrosion and it seems a shame after an ally tank and filler.

If you make a reducer from steel it shoudn't be a problem, as it will be fitted between two peices of rubber? pipe so isolating it from the ally tank. HTH Ray

Personally I don't see the problem with your original suggestion. You will never see it from outside, there's no reason it should leak (I've done the same on my radiator connections and they're under pressure), and the step can't possibly cause any problems - it's not like you're after maximum flow rate or anything!

Yeah I've done the same on some of my coolant connections its just that I thought that some of the petrol would start to settle within that slight lip and possibly lead to problems somehow or that the SVA man might have a sulk at it.

I don't think it's anything to worry about, there would be a slight lip where the rubber joined the metal anyway. If you are still worried then just cut a chamfer on the inside of the tube so there is nowhere for fuel to sit.