Or install an electric car charger. Most of them include a standard 3-pin socket, so could be used for bikes.
Most of them are tethered or untethered with a type2 socket, in fact I’ve never seen an electric car charger that also comes with a 3 pin socket, altho I don’t doubt that the option might exist. Unless the OP owns an electric car they won’t get an OLEV grant (altho other grants exist for businesses that want to offer EV charging), so it could end up quite expensive to buy and have installed. My Rolec untethered ev charger (without 3 pin option) cost £600 installed. Rolec also offer a separate outdoor 3 pin unit for electric mower, hedge trimmers etc that you could plug an ev into, if that’s what you meant?, but that’s just an IP rated outdoor 3 pin plug box.
Anyhow, as said, the electric will cost pence for the realistic length of time they’ll stay around. Personally I wouldn’t spend any money on the service tho, I’d just offer the 3 pin sockets. Assuming they’re buying coffee and cafe. Maybe with a disclaimer in case your ‘various electrical problems we’ve had lately’ kills their battery