Excess is likely to be something around £100 (being market average) unless you requested to have a greater value excess in return for cheaper premiums when you took out policy.
M&S do provide a no claims discount, which at best is usually something around 30% off your annual premium. If you decide to claim it only steps back, rather than disappearing completely, probably it will go back to around 20%, if you have had 6 years since your last claim.
What does your policy cover, most have a limit of £500 (per bike) unless you have specifically mentioned the bikes at inception and increased the value for which you insure them.
Therefore, 2x bikes stolen value circa £500, pay £100 excess, perhaps loose £50 in discount over yearly premium, leaves £350 reasons to still proceed with a claim.
View it from an accountants point of view and it makes sense to use your insurance if the above guidelines sit with what you’ve got as your policy.
market is very competitive, M&S gets good press over cover they give for MTB, Halifax also is equally strong – I insure £4500 5 spot (mainly XTR build) and £2500 Prince Albert and premiums are very good c/o Halifax.
Hope this helps.
I am a claims person for my job.