Do I have any say in who my viva external examiner is or is it completely up to my supervisor? Who pays for the external's travel - my supervisor or the department?

(this is a bit engineer-y, don't know if anyone will understand what I mean) Is it okay (allowed) to put mechanical drawings of commercial apparatus (say from the company's website or catalogue, or the datasheet/manual that came with the apparatus) in my thesis? I'm just a bit worried about divulging design info. that the company is protecting... There's one particular fact a company told me, which is really important for a discussion in my thesis, but they told me it 'in confidence' as it were.

You should be able to discuss who your external is with your Sup. He may suggest 3 suitable people for you to choose from. However it may be that there's only 1 expert out there in your field. Not sure who pays - maybe external, as Sup would do the same, in a reciprocal arrangement?

I would put 'public domain' drawings in Appendix. Obviously if confidential, keep it that way. However there have been PhDs working for MoD, which are top secret with an external from MoD, etc. So if your PhD is confidential, it may be okay to divulge company secrets - ask your Sup.