I've found it to be the complete opposite to that MMSterling. Obviously if an original band makes it big then they're going to be making the big money, but at just a local level the cover bands make easily more, or at least from my experience that's the case. For example, I'm from Leicester like you are (well, Melton Mowbray, near enough), and when I was in original bands playing at places like The Charlotte and The Shed we were only getting about £2 per person we brought into the place, so even if we managed to get a coach load over there we'd get about £100 tops for the whole band. Now I'm doing a cover band and we've been getting around £150-£200 for playing in pubs, plus we're trying to get a few wedding and party gigs where you can get some really decent money, e.g. we're doing a wedding early April for £500. We havent been gigging for that long, but already we're playing gigs more regularly then I ever was in any of my former original bands. You're obviously right that an original band has greater chance of making it big, you don't get cover bands getting record deals really (unless they're a rubbish boy/girl band of course).
That's just what I've experienced over the years anyway, as have a lot of my mates who have been in original bands, and that's one reason me and a couple of mates started up the cover band, to get more regular gigs and get paid more, which has been the case so far.