Hi,

What is it that would cause the instruments to go mad or just not work on a 2000 reg Octavia?

A fault of some sort

Have a search, there is an issue with earlier models clocks going mental.

dry joints on one of the voltage regulators would be my guess.....

Silver Rocket - obviously!

mbames - I thought about a dry joint somewhere but I wasn't sure where to look. I guess these rae behind the clocks?

There is a known-ish fault[1] with the dash pod circuit, where pulling out fuse 14 (iirc) cures it, but causes trip information to be forgotten when the ignition is turned off. I believe the regulators are on the circuit board of the dash pod, but getting to it can be a bit of a PITA.

Your fault sounds less serious, but may well be the beginnings of the same issue.

[1] guages all drop to zero when driving along, and then spring back to life, and then drop to zero... and repeat.