Thanks for the responses, guys.

Lonnie's sound actually got progressively more 'electric' as time went on. The first two singles (Rock Island Line & Lost John with their respective 'B' sides) were recorded before he first went to the USA in '56 and were definitely fully-acoustic, even if 'Lost John' had a distinctly 'rock' feel.

When he heard that 'Lost John' was in the top ten, he returned to Britain to capitalise on the situation. Pye wanted an LP, so Lonnie hastily assembled some song-titles and recruited Nick Nichols and Micky Ashman on drums and bass respectively. The story surrounding Denny Wright's arrival is more complex, but he was called to that first post-USA session with, according to Donegan, a borrowed Hofner Committee (his guitar was out-of-town at a gig he was doing) which was played acoustically throughout the record date. Listen to 'Lonnie Donegan Showcase' and you'll hear that clearly there are no amps in the studio.

'Bring a Little Water, Sylvy' was also completed at the same time (as well as, I believe, 'Dead or Alive'); Denny's solo on '...Sylvy' was actually on mandolin, even if it's almost inaudible on the recording.
Listen, too, to Denny Wright's excellent piano accompaniment on 'How Long, How Long Blues'. Lonnie does a great vocal on that one, too!

'Don't You Rock Me, Daddy-o' had Denny sounding a little more 'electric' (as he also did on 'Cumberland Gap') - I believe he had a preference for unobtrusive amplification (more like sound- reinforcement) and liked the acoustic sound of his guitar to prevail - this is in common, of course, with all jazz guitarists of that time.

Jimmy Currie (Denny's replacement) was much more of a rock-player, but still with a fine sense of jazz-phrasing. His amplified sound is much more apparent that was Denny's.         

Les Bennetts was the clumsiest player of the three, but I've always had a sneaking suspicion that Lonnie actually liked his flashy and gimmicky style - at least in commercial terms. To me, it was the end of an era, though.

Throughout this period ('56 - '60) the overall sound of Lonnie's records got slightly more abrasive and more 'pop'; this was, no doubt, deliberate.

So - the important thing here was Donegan's professionalism. No doubt his band sounded much sharper than McDevitt's or The Vipers, but I have no doubt that this was primarily due to the high level of competence at work on Lonnie's records - Donegan himself very much included! If anyone needs affirmation of this, listen to '...Daddy-O', 'Cumberland Gap' and 'Dead Or Alive' - remember that Denny was also very much an instinctive arranger - these tracks (and others) stand up very well today in terms of dynamic excitement and musicality. Observe, too, that Denny then took his talents to Johhny Duncan's Blue Grass Boys - his is the guiding hand behind the wonderful arrangement of 'Last Train To San Fernando', arguably the only non-Donegan 'skiffle' recording that really repays a listen in the 21st Century!

In summary, I can't quite agree that the electric guitar was the difference between Lonnie's work and all the others - the electric guitar was just one of his musical tools - the dynamism of his performance was the real catalyst....