Finally got it sorted, having taken out the DRO a couple of times, cleaned it, replaced battery and still had problem I went out and bought new battery (LR44 Duracell) and it now works ok.
Seems that I must have had some duff batteries, probably some cheap generic type. When I come to renew again I will change to SR44 type as Ketan advised.
Many thanks to Ketan and his colleagues for phone calls to help me on this. First class service from ARC as usual.
Bill D.