Rich, have a crack at it yourself, it's fairly straightforward, will kill a little time whilst work is slack!
The way I did mine was with fibreglass loft insulation, much better than that silver stuff or polystyrene and easier to use too. I think it's best only use 50 or 75mm - cramming more in defeats the object because compressing it loses the air space between the fibres, which is what does the insulating.
You'll need:
1 or 2 rolls of fibreglass depending on size of van (my MWB transit took almost 2 rolls)
6mm ply for sides & roof (any thicker doesn't bend enough to follow curved sides)
9mm ply for back doors & bulkhead (better to screw into for mounting tools etc)
Battery drill (preferably 2 batt drills or an electric screwdriver otherwise you'll spend all day changing between drill and screwdriver bits)
pilot bits (have at least 3 cos you're bound to break 1 or 2, I think I used 3mm dia drill for no.6 screws)
Right length screws (I think I used no.6 screws, 1/2" for the 6mm and 3/4" for the 9mm)
A sharp hand saw
A big bench or 2 saw stools to cut sheets on, or 2 wheelie bins at a push

circular saw helps for straight cuts but hand saw will do
jigsaw to do fiddly bits
clamps
stanley
scrapers
sandpaper
permanent marker
6 pencils cos you'll lose 5 of them but don't worry they'll turn up after you don't need them any more

This is a 2 man job, doing it on your own sucks.
First, unscrew your existing ply lining and cut/insert insulation, being sure to fill all the space. Then ply your roof, it can help to temporary fix the insulation with tape so it doesn't slide about. Then do the bit between the roof and existing if there's a gap.
If you've got those hardboard panels on poppers covering hollow door sections, take them off, insulate and ply over the top.
Loads of fannying about measuring and cutting out notches and holes for lights/switches/handles etc but all fairly simple.
Grip your drill bit in the drill so it only just goes through the ply and steel and doesn't pop through the outside skin of the body work! Always check you're fixing to a "hollow" part ands check you won't hit the outer panel!! use a wood block as a stopper if unsureWhen all your panels are fixed, you can tidy it up by running gripfill along the joints, rub with damp rag, sand off when dry.
If you want to completely seal off your load area from the cab, any stupid awkward holes etc you can fill with expanding foam. Just use gaffa tape on the other side of the hole you're filling otherwise you'll fill your cab up with foam

Let it dry and scrape/sand it back.
Undercoat all round, then a couple of coats of gloss. Use a mini roller they cost a couple of quid, thin the paints a bit to roll them.
JK sells stuff called protectacote, horrible to work with but waterproofs well and is non-slip, very effective, apply 2 coats to floor and a foot or 18" up the sides too. Treat it like radioactive waste, don't touch it or you'll have to wait until your body sheds its skin, it doesnt come off! Get some xylene off a guy on ebay, it's the only thinner suitable for protectacote, costs about a tenner. Needs to be a decent surface temp for putting the stuff on though so leave a heater shut in there on max overnight before applying
Alternatively, if you're feeling flush, send it to Alltec who'll charge you about £700+vat to ply line it for you

And then get them to coat it in speedliner (similar to protectacote), cost you another £600+vat

You can see why I did it myself, set me back about £270 in total but had some insulation free so would have cost a little more.