Peter,the CWGC Memorial Information mentions:-
The Panel (or Bay) numbers quoted at the end of each entry relate to the panels dedicated to the Regiment served with. In some instances where a casualty is recorded as attached to another Regiment, his name may alternatively appear within their Regimental Panels (or Bays). Please refer to the on-site Memorial Register Introduction to determine the alternative Panel (or Bay) numbers if you do not find the name within the quoted Panel (or Bay).
Just a hint for Jim or others that there may be a different location shown in the Memorial Register..... In your case it could be that he was attached to another Regiment at the time of his death.
As mentioned, "Attached to the 7th Battalion Westmorland and Cumberland Yeomanry" but may be either on that or the Border Regiment Panel (if that's not Panel 1!!)
I suspect that's why he's not been found on Panel 1 so far, he's on a different Bay due to be "detached" to another Regiment and should be found there...
Name: ATKINSON, HERBERT FAIRER. Rank: Private
Service No: 260487. Date of Death: 25/03/1918. Age: 23
Regiment/Service: Border Regiment. 7th (West. and Cumb. Yeomanry) Bn
Panel Reference: Bay 6. Memorial: ARRAS MEMORIAL
Additional Information: Son of Anthony and Catherine Isabella Atkinson, of Selside Hall, Kendal, Westmorland.
So, it could be Bay 6 where he'll be found..... with the other 37 men from the 7th W&C Yeomanry
Hope that helps get your photo!!