Just to update here (may help others) having spoken by phone and chat to Motability
If buying private new/used car and you get it before Motability car handed back you pay the vehicle tax , but once handed back you can claim remaining months back , when you change the cars tax status to disabled with your Certificate of Entitlement
Claims Letters
“Claims History Letter can be sent at any point in your lease and doesn’t have to be at the end” only full years with be on the letter.