I've found a company that can weld my ali tank for me and I'm keen to get this done next week. However I have not decided which fuel filler cap I'm going to use yet - do I need one to know what diameter to make the neck on my tank that the filler hose will connect to? Or is there a standard diameter?

Also, what is a fuel nozzle insert, and why do I need one fitted to my unleaded vehicle for it to pass the IVA?

I'm thinking now that I might neglect to have an external fuel filler cap mounted on my rear panel and use a motorbike style fuel filler cap mounted directly on top of the tank.

More boot space this way as there is no filler hose in the way, but maybe a bit annoying to get to when the boot is full. Thoughts?

Some fuel filler caps (aero style) have dual size built in so pick you cap then get tank neck made to fit
If I remember sizes are 55mm and 50mm last being most common
As you can't buy leaded fuel any more I think the restrictor flap thingy is not needed
Car Builder solutions have filler caps and dimensions

Since my tank setup have rubber hoses in at the rear, changing to copper through the tunnel and back to plastic exiting at the front firewall, I decided on the following system to eliminate high pressure in the fuel line from the tank that can leak (fire risk) at the joints in the line.

At the tank I installed (any suitable place) an inline low pressure pump that move the petrol to the front and dump in a surge tank. At the surge tank I have four nipples fitted for;

1. return fuel line to tank when surge tank is full (on top)
2. Return fuel line from fuel injection rail (on top)
3. Supply from the low pressure pump at the tank (top side)
4. Supply to high pressure fuel pump (bottom) which result in the fuel pump always gravity primed in my setup

The beauty with this is that the fuel line to the surge tank is open ended so to speak and now pressure in it which is safe (just circulating and no pressure in the surge pot). The only high pressure pipe is the little distance from high pressure fuel pump to injection fuel rail. Another plus with this is the fact that the high pressure pump cannot surge cornering etc.
The con is you have to have two fuel pumps.

See this picture taken from my car's setup.

Thanks for the info. Interesting set-up Wynand. I've already gone and got an in-tank fuel pump from a Mondeo so I'm going to go with that, just make sure my connections are strong enough to take the pressure.

One last question - how have people done their tank vents? Will a one-way valve be enough to allow air to enter the tank as the fuel level lowers, or does it need to be two-way to allow for expansion of air on a hot day?

I'm pretty sure it must be a two-way affair. It must allow air in when the petrol is being used, and it must allow air out for thermal expansion. I just used a 1 m length of fuel hose, from the right rear corner, where the breather port is in the tank, it runs up to the chassis, then left towards the opposite corner and down. Previously it used to be just up and down, but that used to spill fuel on heavy cornering.