Hello experts, what do I need to fix this? A bolt? What specification, and who has them?
Try Peter's Spares https://www.petersspares.com/ (https://www.petersspares.com/)
Try contacting Bachman UK - if they don't have spares they may be willing to tell you what thread, etc you need.
Edit: Copied from the instructions for the Baldwin on the Bachmann website:
QuoteSpare Parts
Selected spare parts are available from the Bachmann Europe Service Department. For further information and enquiries please contact the Service Department directly by email on service@bachmann-europe.co.uk or by phone 01455 245575.
Thank you both, Andrew and Julien!