I've found a couple of links for you
https://www.marshals.co.uk/
http://www.motorsportcircuits.co.uk/...a_marshal.html
https://www.msauk.org/Get-Started/Volunteering
http://www.volunteersinmotorsport.co.uk/grading.php
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And this from the MGCC http://www.mgcc.co.uk/motorsport/marshalling/
Get an MG3 and be prepared
http://thumbsnap.com/s/Glk40p4H.jpg
Pleasure to help you :thumb
I never went on a course myself, I suspect existing Marshalls will train you, unless you start going down the registered route and then I don't know. I guess it's in one of the links somewhere.
i know the castle combe Marshalls had to buy their own orange Marshalls overalls though :(