My new Dash2 is having problems reading anything sensible from my old Smiths fuel sender so I think the time has come to replace with a known new sender.

Problem is.....Since this is a Phoenix the fuel tank is under the bodywork and it looks like I will need to drain the tank and drop it before doing the work. Which brings me on to my question...

I didn't build the car (or any kit so far) so I have no experience on how the fuel sender is attached. Obviously it has a number of bolts (6 I think) which must have nuts of some description on the other side. I presume that these are captive in some way (?) otherwise when I undo them the nuts will fall into the tank..... or not undo. I appreciate nobody can actually tell how this one is done but how is it normally done in a kit ?

I think you will have to drain it and drop it.
Most tanks will have captive nuts or studs.
If yours is a 4 stud which the Sierra is then in my case the four studs had a bigger PCD than the normal 5 stud so make up an adaptor plate with 5 studs and the appropriate hole, the gauge fits in the adaptor and the adaptor fits to the original hole and fittings.
You will need to use a seal for both the new sender and the old hole in the tank.

My vdo sender has a ring with 6 holes tapped into it, that fixes on to one of the bolts and is then inserted inside the tank and the remaining bolts are fitted and tightened down