When I use my Beovision 6-22 with an NTL (Pace cheapie box) STB, I am getting a lip sync problem. The Beovision works fine with an analogue signal.

While I suspect the NTL box is the cause, I have another Avant that is using a similar NTL box without any issues. 

Quite a common problem and certainly nothing to do with the Beovision 6-22

This tends to happen for one of the following reasons:-

1) The Pace STB has a fault or needs a software update

2) The aerial signal is below 60% which would allow you to still have a reasonable analogue picture, but is likely to provide picture and sound mismatch on digital -- unlikely if you are using the same aerial for your AVANT which you say is fine.

I think the best first action would be to use the STB which is working on your AVANT and put it on the BeoVision 6-22 and if it works without problems, then you know you have a problem with the Pace STB