My ignition barrel failed so I fitted a second hand one. I had a new key shell built with the circuit board from my old key, and the key blade for the new barrel. The car now unlocks and locks fine, the ignition barrel turns around but the car won’t start.

Can anyone help or recommend what to do? Thanks

Did the immobiliser chip get moved across from the old key? That's not on the circuitboard, nor in the key blade. Small glass tube a few mm long. Needs gentle removal.

I don’t know. A shop cut the new key for me. If this glass tube is missing, and is the issue, can I solve this?

If you no longer have the carcass of the old key then you have problems, do you have a spare original key that used to start the vehicle?

If so then use the chip out of that and have a second cloned key made at the same time.

Was the inductive reader coil around the ignition barrel disturbed, removed or disconnected?

The car originally broken down and the RAC pulled the ignition switch out to start it with a screw driver turn.

This disturbance might be the issue.

Is this a job for an auto electrical repair?

If you like throwing good money after bad then yes, if you don't then its the time to invest in VCDS.