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Drain the old petrol and let the tank "breath" in a warm dry place (That now smells of petrol) and it should lose water out of the plastic, returning to a more normal shape.

There are recommendations that you use ethanol free petrol to avoid this, but that's less easy for you as you are in Devon.

The Norton Owners Club included the following comment on its website last year:

'Esso super unleaded petrol (Synergy Supreme+ Unleaded 97) is ethanol free (except in Devon, Cornwall, the Teesside area and Scotland) and we have no current intention to add ethanol to Synergy Supreme+ in other areas of the UK.

We would therefore advise anyone who has concerns about the presence of ethanol in petrol to use Synergy Supreme+ - providing they do not fill up in Devon or Cornwall, the Teesside area or Scotland.'