Using comparethemarket and the site above, the excess insurance doesn't work for me at any level.
If I take £0 excess it quotes me £233. £500 is £210, so £23 saved - but £250 excess cover is £28 and £500 excess cover is almost £35. Quote provider is the same except the last one. All have £250 compulsory excess except the last one (zero excess) which is only £150 compulsory excess (and happens to be the one I have).
I realise it would be very dependent on personal data.
Thanks for the link though, I'll suggest my daughters have a look next time round as it may be better for them.