OK here's an up date.
It turns out that changing the battery is quite simple. It is a 3.6V 120mA/h Ni-mh cell. The only drawback is the physical size. The type fitted in mine was about the size of 2 button cells one on top of the other although the PCB is marked to be able to take a flat pack instead. The original doesnt seem to be available on the net as far as I could see.
There are two other easy options. One is to use 3 AAA ni-mh cells in series which will just about fit inside the case the other is to use a VARTA 55615403059 pack (Farnell/CPC).
Apparently despite what i was told earlier you can still get the original from Seaward but it's seriously expensive for what it is.
To get to the battery you need to gently peel back the lid/display overlay. This is done by lifting one edge, preferably near the bottom right hand side of the panel, with a very fine piece of metal like a BLUNT scalpel blade.
Gently and slowly peel the overlay off from right to left making sure that it doesn't split around the display window.
There will now be a transparent plastic cover visible over the main PCB which is held in place with 8 screws. Remove all these screws - 4 allow the removal of the plastic cover and the other 4 allow the PCB to be dropped forward from the case.
The old battery can then be unsoldered from the PCB and the new battery soldered either directly to the original holes or with fly leads to the pcb.
If it is NOT the original style of battery you may want to cover it in heat shrink tubing and then, after reconnecting to the pcb, stick the battery pack to the board with a drop of glue or a small strip of double sided tape.
Check all your connections, power it up and there you go - one replaced battery in a PAT1000X. If you want to check then put a meter across the cells and ensure that there is a voltage of around 3V or so which will be gradually rising as the battery charges.
Now as for refitting the front panel overlay there are two options.
One is to reassemble the board and cover to the case and HOPEFULLY there's enough adhesive still on the original overlay to simply restick it. (New overlays are available from Seaward but they too are pricey.)
Another option is to scan the original overlay onto your PC and create a new one by encapsulating a printout with a document sealer and then cutting it to size, adding some contact adhesive and CAREFULLY gluing it to the panel.
OK the PAT1000X is old but it's still ideal for someone like me who only checks his own workshop gear. Scrapping it and buying a new one was not an option simply for a failed battery and now it works perfectly again.
I hope this might be helpful to some other 1000X owners