Apparently, according to a broker that I know, DL and Admiral control a fair chunk of the market between them. The whole sector is a sham, and ripe for the sort of mis-selling investigation that happened with PPI - there's no effective competition, no transparency and a range of pretty dubious anti-competitive practices across the board.
As a related example, my RAC policy premium - which is an insurance policy, just a different type - jumped by more than 50% on renewal, and I had to cancel my policy, leave the RAC after 17 years, leave a shi**y (but accurate...
) review of how they handled it before they responded properly.
End result: policy renewed by them at the same price as last year, but what a waste of my time and their management time, and so typical of the sector as a whole.