Dont give up on m&s just yet.
Had the renewal quote through yesterday. Insurance premium up from £533 to £617, obviously with the new lower bike limit. No excess.
Just off the phone to them, raising voluntary excess to £200 was the sweet spot for premium reduction, bought it down to £379. Adding one bike at £4000 valuation took the premium to 460, another £80.
Happy with that since there's another three bikes each worth 1000 -2000 still covered. Changes to cover as noted above- external buildings need to be locked, or the bikes secured with a locking device to motor vehicle or immovable object.
A lot cheaper than specialist providers, and I'm a lot happier having new for old cover in place.