My 04 540 came with some spare alloy wheel bolts, but of course the spare wheel is steel rim. So I need to get hold of some steel rim bolts in case of a puncture.

On the basis that they are probably a standard bolt, on the off chance, does anyone have a bolt they could photograph and provide dimensions from please so I can get the right length and size?

The size you need is 12 x 1.5mm R13 26mm thread length. The R13 bit is the radiused seat used on original Eriba steel wheels.
I recently purchased a set of these but with the tapered seat to suit a pair of wheels I got with new tyres (kept the originals for winter storage).
Bolts seem to come with 17mm heads in the UK rather than the original Eriba 19mm steel bolts. Doesn't make any difference other than making sure you have the right socket to fit them.

Here is is photo of the original and the tapered seat alternative (black one).

IMGP4933.JPG - Bild entfernt (keine Rechte)

Thank you very much indeed - that's just what I needed

Just to square the circle - nice quality 19mm AF arrived today 2x4.

https://www.westerntowing.co.uk/product/...m12-spherical/#