Hi, firstly thanks to all of you for your input it is as always
greatly appreciated.

I found the microswitch. Once I pushed it down I can hear a definite
click which removes the fault from the panel and leaves the tamper
light on.
When I put the cover back on the fault light re-appears thus
preventing me from clearing the tamper light.

I looked on the other working PIRs and I dont see what keeps the
tamper switch depressed. i.e. when the cover is lifted it would raise
and then show tamper.
So I cant seem to work out how that switch is actually activating.

Andrew- In the past when i replaced the battery on the PIR it would
show tamper and then I would just need to put in the user code and it
would clear. So in that case it didnt need an engineer code.

CJ - I couldnt see a rear tamper (I removed the whole PCB from the
casing to look at the back. On the front part of the PCB it has a 4700
sticker so I assume that maybe the 4700 and 4705 PIRs differ in this
respect.

I've put some pics here
http://www.flickr.com/gp/71954231@N00/i61g8V
if you have any further suggestions...

....although I'm getting the feeling that the new TV will have to wait
a bit longer until a new alarm is fitted