I am in need of a length of thick walled (2.5mm+) 12mm i/d pipe/tube to make up the mountings for my new engine cradle. Can anyone recommend a supplier, as I am having real trouble finiding something suitable....

Try having a look at Commercial/Industrial heating suppliers.

Such as Pipeline centre, BSS, etc.

1/2" Mild steel tube with BSP threads is often used on bigger heating systems etc.

Two thoughts:- 1) can you not use 25mm square section tube if you have some left over from the build?
2) I have some 25mm O/D CDS tube 3mm wall thickness i.e. 19mm I/D if you need short lengths for a small donation + postage.
Cheers, Pewe

It is to support the M12 bolts, therefore, only a snug fit will do - but thanks for the offer!

I have found some old heating pipe at work - removed the paint from the outside and it seems to fit the bill... The strange thing is I had exactly what I wanted already, and thought it would be easy to replace. I practiced a few too many times with the welding, and now I have none left!