Your pretty limited in the Portsmouth area for good mtb shops
Most shops are road/commuter/general bike type shops
We do have Evans as you said along with the following
Gabriel Cycles in Widley (just over the top of the hill from Cosham) they are mtb and e-mtb based but also do a range of other stuff, they are prob your best bet
Hargroves in Chichester & Southampton (Fareham branch has closed down)
Cycleworks and Owens cycles in Petersfield
Solent Cycles in Fareham, lee on Solent & Whitely
We did have Velocity/Southdown bikes in Portsmouth/Cosham and The Riders’ Guild in Chichester but all have now closed down
Even Wiggle have now completely left their old building on the Southampton road and only have office staff at the Lakeside complex, miss the days when you could walk in to their old sites to collect your order, even remember when they were just Butler Cycles in Southsea
Cycle surgery in Port Solent do some mtb stuff but are mainly road/commuter based