Hi Dave,
The 3rd Militia Battalion was formerly the 1st South Durham Militia and were headquartered at Barnard Castle since 1853. They became The Durham Fusiliers in 1868. They were the only English militia regiment to convert to a Fusilier regiment. The Officers wore a silver flaming grenade badge on a fur fusilier cap. One is illustrated as part of the collection at the DLI museum and was worn 1868-1881.
www.dlimuseum.durham.gov.uk under objects - hats.
Regards Dean.