If you can't get electric to the van at night to run a frost heater, then the propex gas heater is the way to go.
The diesel air heaters draw a high battery amperage to get fired up so drain the battery. On a cold night they will stop and start many times which is the problem. A propex heater just takes minimal current to ignite the flame and then use a small current to run the propex fan, as will the diesel heaters.
Diesel is convenient, but also expensive. A diesel heater will draw about a third of a litre of diesel an hour, so if it runs for 9 hours a night, then expect that to cost around £5 which may extend to the day as well.
If you invest a little more money, you can buy a refillable gas bottle. This has two advantages - the gas from a fuel station is cheaper, and if you get the transparent type, you can see how much gas you have left in the cylinder. (I would check that your chosen service station does allow you to refill these bottles. They were quite strict about this at one time do to people trying to fill ordinary gas bottles which is very dangerous.)
The downside is that you need to check the implications of gas with your insurer and it becomes a little more difficult if you haven't an LPG gas supplier near you.
I would also drill a vent hole in the floor of the van as a safety procaution. Gas is heavier than air so if there is a gas leak, this is a good way to let it escape.
There are a number of users on here who have successfully used these heaters in winter - Roy Harding is one.
A mate of mine is unable to heat his van at night, so drains his pump and hoses down every night in the winter. He takes his hose reel and pole inside to stop them freezing. He has always been able to work in winter when conditions allow.