I'm just letting my brain roll around for a moment.
I know that Sterling products are the most superior kit available but;
Sterling list their 60 amp unit (BBW 1260) for £344.90 including VAT and the 120 amp (BBW12120) for £499.90 including VAT.
Looking at the description on your website of "12v 60/120 AMP Battery to Battery Charger Kit," I wouldn't know which unit it was. However, your part no infers that it is a 120 amp.
If it is a 120 amp then something tells me your pricing of £240 + VAT doesn't sound right. (With your markup included in the price of £240, I can't believe that Sterling would give you a discount to achieve that. 10 years ago we could only get 15% from them as a commercial new van dealer.)
I see you don't have stock.
I would consider installing a solar charging panel on the roof of my van to trickle charge my leisure battery and supplement that with a bench charge as needed. My opinion though.
But I'm afraid that the description comments regarding the issues with kinetic recovery are going to affect us more and more in the future. Motor manufacturers are being required to police the vehicle emissions legislation by making it difficult to work around.