Hey Lee,
When do they blow the fuse? When you open the door, turn them on manually or just straight after replaing the fuse?.
You got just the standard lights or have Balfour added anything extra?
If the van came straight from Balfour they probably had orange rotating beacons on it. Are these still on it? If not then they may have just taken them off and left the wires trailing somewhere, these could be shorting against the bodywork and blowing the fuses.
My van is ex-police and when they decommisioned that they just cut through everything

Nothing worked when I got it.
It's not hard to imagine Balfour might have used the interior light circuit to power anything extra they put on as it's so much less work but is very bad practice.
Steve