Update for interest:
Last year my Santa Cruz Juliana was stolen from my shed (brick outhouse not a flimsy wooden thing) and I received a successful payout without fuss from M+S Home Insurance.
I have been dreading the renewal thinking they would either whack the premium up or refuse to insure our bikes (two FS MTBs and 2 road bikes) but the renewal arrived today and it was pretty much the same as last year and the £4k limit remains. Did a quick price comparison and managed to knock another £20 off and get £45 cashback from quidco, so buildings and contents insurance is costing about £230 for the year including worldwide cover for getting on for £10k worth of bikes – very happy with this!