digital clock gains time

would it matter if i went for a higher voltage rating?

No, if it will fit. Higher voltage usually means larger physical size.

Taking the case off and it then keeping proper time, might suggest it is temperature related - so possibly a capacitor changing value as it warms up.
 
just to update on this. i replaced all the caps and the larger electrolytic seemed to be bulging at the base compared to the new one. it now keeps time so will go back into service as my alarm clock.
thanks again to all on here who took the time to respond and provide info/links
 
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