Maybe the hole in the petrolcap is blocked.

There is no spacer or nut between the frame and stay check your frame against the seat.
At the rear of the frame, no spacers between frame and mudguard, that would mesure the frame.
Adrie.

Hi Jim,
maybe your 1950 Ariel never had a trouble light.
Adrie.

Hi Alan,
the twin inner case is offset at the crankcase side, where the singles are in line.
The outside deck is the same as single.
Adrie.

Hi John,
try a heath blocker between the head and carb.

Adrie.

Hi,
take the tank bottem out, sandblast the stuff away, repair the leaks, weld the bottem in again, forget the sealant.
Kind regards Adrie.