Thank you for your reply.
The vehicle reg is HN13 NKT
I will contact the dealer today and see if they have the voltage readings.
We are a flooring company and the van is used to carry goods to site and then normally parked up for the day while working on site, some days several jobs on and would be more stop starting, but the starting problem seems to arise when the van is parked up all day on one site.
Also we so far have not had a issue during the summer months.
The mileage is 15532