So I decided I'd have to start from scratch. I knew it was a fairly large sized battery and with a rating of at least 95Ah.
The existing battery was a Panasonic SR95D31L and looked as if it was the original as fitted by the manufacturer. I measured this battery and looked around the internet for a suitable replacement. It seemed to me that this power of battery in the case size required was not readily available. Halfords had nothing like it - either the size was too big or the power too small. It didn't help that a Bongo was not on their lists of vehicles.
To cut a long story short I eventually found a Bosch battery on ebay which seemed to fit the bill. (hawk-asg (9880) power seller, feedback 99.7 positive) The specification was as follows:
249/335 BOSCH SILVER S4028, CAR BATTERY 95ah, COLD CRANKING AMPS 830 SAE.
DIMENSIONS 306 L x 173 W x 225 H, (Total height to top of terminals). Also the positioning of the + and - terminals was perfect.
Delivery was next day if you ordered before noon. So very pleased and have now fitted it. Photos below:
Battery tray before fitting

Position of terminals

Battery model etc.

I'm posting this in case others have found a problem sourcing a main battery!