Interesting account here:
https://51hd.co.uk/accounts/tom_garside_st_valeryThat site came from a Google search on the subject '1940 St Valery' and loads came up.
As Max said I would start with his records from the MOD. Then I would get the War Diary to see what that says about events affecting the unit. Other routes are to try to see if there is a 'Missing Men' file at Kew for the RASC of 51st Highland Division that mentions him. You could also see if there is anything in any of the number of books on the subject (get them on Abebooks.co.uk). You could try the Royal Logistics Corps Museum to see what they have and the Imperial War Museum library to see if the have books and unpublished manuscripts from veterans of the debacle.
If you cannot get to Kew yourself there are a couple of people who post on here and WW2Talk.com who provide that a copying service for you at a fraction of what the National Archives costs.
You may strike lucky....I have some times but others there is nothing at all and their fate remains a mystery. But it should fill a few evenings fruitfully!!!! And it is addictive.
Good luck!