I had the same problem, I bought a new speedo from flee bay for £25 posted, and a new speedo cable
Take out the 4 screws that hold the dash in and ever so carefully unscrew the speedo cable from the rear
Next take a picture of the rear of the dash wiring and then label each wire, the new speedo is easily replaced (just 4 bolts ) carefully replace all the wiring
Jack up front of the van take off the near side wheel
Now take the clip that holds the speedo cable to the near side wheel hub (see picture) next tie a length of string to that end, then from inside the van pull the cable ever so gently through along with the string now tie the sting to the new cable and gently pull the string back through along with the new cable from under the van the new cable will follow the original string path, put some grease on the end and push from the back of the hub into the square retaining hole put the retaining clip back on, connect the other end to the new speedo put the 4 screws back into the dash and hey presto working speedo
I did it for the first time last week and it took me about 3 hours ( I had a problem with the earth for the dash lights but everything ok now)
Good luck
Bazza