Hello Dave.
When we had an outboard engine we had to plan our trips well in advance for petrol stops. We found the large supermarkets along the canalside the most reliable. Tesco, Sainsbury and Asda were the most popular in our experience. We went into our local chain, of each of these stores, and picked up a leaflet from customer services, detailing where their stores are nationwide. These leaflets state which stores are 24 hour, which stores sell fuel, which have a pharmacy, etc. They are very useful leaflets. A 20 litre gerry can and a lightweight folding trolly are also useful.
Both Nicholson and Pearson guides show petrol on their maps, but of course they quickly become outdated as garages close down, and new ones spring up.
We also have a Garmin Nuvi 300 SatNAV which we find useful, especially on "pedestrain" mode, because this shows the canal, and Points of Interest, shops and petrol stations, etc.
However, we no longer have a petrol outboard, having changed our boat to a diesel inboard.
If you are planning a particular trip let me know, and if we have covered that route I may be able to help you locate fuel.
We covered many miles with an outboard, including Llangollen, Manchester, London - all starting from Staffs and Worc canal.
FirstMate.