Hello
Following on from advice sought here and elsewhere, I went about it thus:
1) Ordered 300 feet of 1 x 1.5" sawn battening to sit along the tops of the existing joists2) Ordered twenty 8'x2' 22mm tongue-and-groove chipboard panels (heavy duty)
The reason for the battening was so that I could rebate the lengths where necessary to allow for the various bits of twin and earth that were fitted across the original joists - and avoiding notching out the actual joists. I decided on the big loft panels because I reckoned they'd be quicker to lay than the small "B&Q" loftpack boards, and they're 22% thicker (and hence they are stronger).
It took us a day (two people) to lay the battening and notch out with a pull-saw and chisel and rearrange the mains cabling into the channels. It took about a day and a half or two days to lay the loft boards. Where the ends of the loft boards miss the joist (the boards are layed perpendicular to the joists), we used PVA wood glue. I've screwed them (and the battens) down using chipboard screws (twinthread gold screws) and a Makita battery drill/driver which did the whole lot on a single charge.
Last time I did a loft of similar size I used the B&Q packs and although it went OK, I had to cut most of them to size on accout of non-standard joist spacing in that house. That's what made me decide to use the bigger boards - they span more joists per board and they're that much thicker that, when the ends miss a joist, a decent amount of gluing makes those ends adequately self-supporting.
If we ever do it again I shall use the same approach but I might cross-batten instead of parallel batten. Cross battening will implement sufficient clearance for the mains cabling and will also lend strength to the structure by adding a bit of additional support at the board ends.