I'm very non-techie so this may be confused....
Just bought an HDR 2000T and it seems to be set up OK.
But I was shocked while watching one channel to find it suddenly switched over to a channel I had scheduled to record.
I hit the channel number I had been watching but a screen notice said the channel could not be changed from the one being recorded.
So I then had to hit 'input' on my TV remote and choose Antenna to get back to the original channel I was watching.
On my previous Humax I could switch between channels even when one or two was being recorded
Have I set something up wrongly or is this how I must do things in future?

All of the Humax HD PVRs can record -1-watch-1, record-2, and record-2-watch-1 (as long as two of the three channels share a common broadcast channel).
However if your HDR-2000T was recording two channels on different mux, and the channel you wished to view was on a third mux, then it would fail in the way described.

You need to test to clarify the situation, that if the unit you have obtained is faulty you can seek replacement before recording programmes to it you would not wish to lose.

Remove and reconnect the power, to be sure the PVR starts up to viewing only.
Select channel BBC ONE (SD, regional) and start an instant record.
Then switch ITV1 SD (which is on a different mux), if not faulty you should be able to view whilst BBC ONE continues to record.
Start an instant record on ITV1, then change channels to see which are available. You should be limited to BBC SD channels, and some of ITV, Channel 4 and Five groups.

If ITV1 does not display, reset the PVR to factory/defaults setting and set it up again - I'm not sure of the HDR 2000T menu but if it facilitates manual tuning use that.
Then retry the tests above.

Thanks so much Nvingo for this important and useful info.
I will do the test you suggest

The Humax in line with most PVRs has two tuners. This means that it can tune into two separate multiplexes, which is not necessarily the same as tuning into two channels. Each multiplex (mux) contains several individual programme channels. The tuner effectively "sees" every channel in the mux while the following stages extract and decode the individual channel. Therefore if both the channels you want to record are part of the same multiplex then only one tuner could be in use.

This then leaves the second tuner free to view any other mux you want.

Alternatively if you have set it to record from two separate muxes then both tuners would be involved and in theory you could watch a second channel on either of those two muxes whilst recording the two programmes. But what you can't do is try and select a channel carried by a third mux as there is no free tuner available.

What might have been the case here is that you had a recording going on mux 1, were watching on mux 2 then kicked off a recording on mux 3. With only two tuners to play with the recorder switched the tuner on mux 2 to mux 3 for the recording. Once switched you could not then return to watching the channel on mux 2 as you've run out of free tuners.

But if you were not recording anything at the time the Humax switched away then that is odd as it should have been able to fire up the second tuner for the recording and let you continue watching uninterrupted.