If I recall correctly, it was because the Basset aircraft had been refuelled with Avtur, not Avgas.
This lead to 'segregated refuelling' at Training Command aerodromes (such as Leeming), which operated both piston and jet aircraft - the fuel bowsers had totally different routes to the flight line and procedures were put in place to ensure that an Avtur bowser couldn't enter the piston flight line and vice versa.