I've no idea why ZM thinks that you need an official form. I ran a railway station for several years and, while the rail company offered forms for the convenience of customers, complaints by letter were handled in exactly the same way as any others.
There are two questions to asks yourself:
1. Did you wait at the correct place? Replacement buses don't always run from outside of the rail station. If they'll have difficulty in turning within the station yard, they might call nearby instead. The calling point for replacement bus services should be shown on a notice at the station. You can also find it by searching for the relevant station details here:
http://www.nationalrail.co.uk/stations_destina tions/
2. Were you there at the correct time? Replacement buses often leave significantly earlier than the train which they're replacing.
If you're confident that you were at the right place, at the right time, it's likely that the train operating company will refund your expenses (but probably in rail vouchers, rather than by cheque). If they don't, contact the watchdog body, Passenger Focus:
http://www.passengerfocus.org.uk/advice-and-co mplaints/
Chris