I soaked the barrel in oil over night and took off any burrs on the shaft. Both slid on each other very well.
So I tapped in the barrel with the aid of the old one and built the pump back up.
There is still a tiny leak – not sure from where, but I think the bottom of the shaft. – it is not bubbling out like before but I will run it in a little more and see.
In hindsight it was probably better to go for a new pump as the spares don’t seem to be up to the job. But I will see how it goes.
Ian