Is the non-return valve (1-way valve) on your car operational? This valve is meant to allow fuel vapour to be pushed out the fuel rails to re-supply the carbs with fuel when heat soaked.
If operational it should make a buzzing/clicking sound. It is also necessary for the fuel return lines to be connected and not blocked for this mechanism to work.
See: https://forums.jag-lovers.com/t/s3-difficult-start-only-when-heat-soaked/352208/8?u=jagwit1