Although when drilling through to SD airfield information we can find whether an airfield offers Mogas/UL91, is there any way of searching which airfields offer these fuels without a stab-in-the-dark technique?

This is the list we use (thanks, UK/GA), but I'm sure there are more:

Ashcroft
Beverley
Cambridge
Cromer
Enstone
Ince
Kiewit
Lelystad
Midden Zeeland
Northampton Sywell
Priory Farm
Sandy
Sherburn-in-Elmet
Uetersen
Wolverhampton Halfpenny Green
Yatesbury

or perhaps a walkable service station from the airfield?

No, this is not currently a SkyDemon feature. I agree we should be able to make use of our data in this way, though, so we will put some thought into how we could do it.

Thanks for the info.
Would it be difficult to add a dated comments field to the Share Fuel Price feature? We could then add the relevant info.

Yes, such an addition would not be trivial at all.