Maybe I should get a spare and keep it handy. However, the boiler won't see much use during the summer and autumn (I won't need any heating until at least September and I use minimal hot water except on bath days), and it will be due for its next service in November, which I may bring forward to mid-October. I might get the 'second firm' chap to replace the tube anyway. I'm not letting the first fitter go anywhere near it ever again.
MM