I had a look on the National Express site, went through the process of booking a random ticket, in the process there's a checkbox to also buy insurance, it was only £1 with the random fare i was fake booking (Edinburgh to Liverpool).
If they still have the ticket or receipt check on it if it says anything about additional insurance.
Note 2 however in the "You are not covered" section might apply to you if the loss wasn't reported to the police.
Insurance note says :
YOU ARE COVERED
A. PERSONAL BAGGAGE
For up to £750 after making proper allowance for wear and tear and depreciation, for the value or repair of your own baggage (not hired, loaned or entrusted to you), which is lost, stolen damaged or destroyed: limited to £100 in respect of all valuables, limited to £100 in respect of a single article or a pair or set of articles (eg golf equipment).
NOTE - In the event of a claim in respect of a pair or set of articles we shall be liable only for the value of that part of the pair or set which is lost, stolen, damaged or destroyed.
YOU ARE NOT COVERED FOR
1) If you do not exercise reasonable care for the safety and supervision of your property
2) If you do not obtain a written police report within 24 hours of the discovery in the event of loss, burglary or theft of baggage, valuables or money
3) If you do not obtain a written carriers report if your baggage is lost or damaged in transit (or a Property Irregularity report in the case of an airline)
4) For loss, destruction, damage or theft;
(a) due to confiscation or detention by customs or other officials or authorities
(b) of contact lenses, dentures, hearing aids, samples or merchandise, bonds, coupons, securities, stamps or documents of any kind, (other than as defined in the money definition), vehicles or accessories (other than wheelchairs and pushchairs only), tents, antiques, musical instruments, pictures, typewriters, portable telephones, computers and/or accessories, televisions, sports gear whilst in use pedal cycles, dinghies, boats and/or ancillary equipment, glass or china.
(c) due to wear and tear, denting or scratching, moth or vermin
(d) of valuables left as 'check-in' baggage
5) For mechanical breakdown or derangement; for breakage of fragile or brittle articles being transported by a carrier, unless the breakage is due to fire or other accident to the vessel, aircraft or vehicle they are being carried in.
6) For baggage or personal property stolen from;
(a) an unattended coach/bus unless it was in the locked luggage compartment of the coach/bus and evidence of force and violent entry to the vehicle is available
(b) the passenger compartment of any unattended vehicle
7) For any shortages due to error, omission or depreciation in value
For any property more specifically insured or recoverable under any other source
9) For anything mentioned in the General Exclusions shown on page 2.
National Express - Lost luggage 
