Thanks for all your advice lads - 
I went for the Yuasa battery in the end as I got a very good deal for one through a friend.  Apparently you need beware of imitation Yuasa batteries  for sale on the internet tho- thats what I heard on the grapevine - don't know how  much truth there is in that  - but wouldn't surprise me tbh.  Will  add another post soon,  to report on  how I get on with the new battery and give my verdict !!