Doe anyone know which fuel sender unit would i use in a robin hood 3A fitted with sierra clocks?

Are you building the car or replacing the sender?

If the latter, can you post a pic of the existing one?
I would have thought if Sierra clocks are being used a Sierra sender would be most likely.

Stu

Robin Hood used a standard sender, outlet and return/breather, 1 wire type.
This works with the Sierra gauges but can be modified to work with aftermarket Euro spec ones.
GBS Kit parts direct do them for about £15.
sealing them is a little difficult, I used 3mm cork and spread the clamping load with bars across two bolts

Ive got a standard sierra fuel sender kicking around if its any use to you.

Cheers

Rich