Could be wiring, on the mark 4 (which later uses the same cluster as the mk 5, I think) it has a few wires under the bonnet, if one is not earthed correctly it could cause the speedo and rev's not to work.
I took of some engine cover brackets in the past and a cable was earthed there, didn't take much notice of it until next day I realized halfway to work my speedo didn't work, gave me a WTF 30min later I realized what I had done, so have a look under the bonnet and see if any cables are loose, disconnected or unearthed. Don't make the same mistake I did and earth it to something that's not metal

otherwise you solve nothing
