Small update - I bought one of the Lidl things today on the basis that even if it didn't work as I would like it with the Volto it would be useful to have kicking around for phone charging etc. So I charged it up and connected it up to the charge socket of the Volto. When I test the Volto on its own I usually get around 2.5-3hrs max from it - that is with a Line 6 Relay unit, Zoom MS-60B, Boss LS-2 and a tuner all switched on, which I wouldn't normally do! But I wouldn't trust it at a gig from set up to the end. Joining this up to it I got nearly 5 hrs, so it is absolutely doing the job I need. So thank you Howiebass!