"...there isn't enough cash going in to run a through life maintenance programme to CAA standards."
That really depends on how many hours they say they're going to fly on their Permit to Fly - The brothers are canny folk and have probably figured it out. I recon they will have a very low 1st year, perhaps 30 hours, that may fund their 2nd year etc.
If the engines are sound and there are no major defects I can't see how this will entail a great cost in the 1st year. Their biggest worry is an engine failure and replacement, off-base.
Admittedly, before the latest reduction is airshows, but I have run a classic aircraft on a Permit to Fly, flying about 80 hours in a year and doing the equivalent of a 50 hour check every 10 hours and 150 hour check annually. Well within the money earned attending shows in UK.
Perhaps their next project is clearing a space to launch from... their launches will all be without 'payloads', so may be a lot shorter that we expect.
Again, I worked at a helicopter base where our new helicopters were forced to do CAT A take-offs, whereas, if they had the length of clear space to move into, they could launch directly from a low hover. They simply bought two adjacent fields and created a suitable 'runway', 700m long, to which they could taxi and launch, with the fullest load.