The good news is the OPs post is still online so I'll post it here to help others
Well, front gas locker door is back in place and now closes and locks better than I have ever known it to!
I won't pretend it was easy to do, it wasn't - took best part of half the day to get it right. If i ever have to do another though, I reckon I could now do it in under an hour. Here follows an attempt to explain how to do it:
From the caravan, remove the rubber hinge and attached white aluminium hinge door mount piece (here after refered to as "Aly door mount" and see first photo) by removing the end screw that does indeed allow removal of the stop end cap (see Copper's post above - thank you Copper! and see also last photo that shows end cap and end screw). This allows you to now work in the comfort of your garage or front room to replace the Aly door mount onto the door rather than trying to manhandle the cumbersome door back into position onto the caravan.
Thoroughly clean the rubber gasket that would sit between the door and the white aly door mount, and set aside.
At this point be sure to be handling the door on a piece of carpet or similar to prevent superficial scratch damage to your door and remember that the door is made of plastic and so will stand only so much pushing and shoving described below before cracking - just be careful.
Now note that the hinge edge of the plastic locker door has a step in it (see second photo). This will cause you pain and trouble as it hinders the re-application of the gasket over the top. Now here is the trick - Take some strong but small diameter string / twine, tie a loop at one end and fit it over the metal locking rod at one end. Run the string to the hinge side of the door and hook it under the piece of unfixed plastic found at the end; run the string along the step in the plastic to the other end of the door, tuck it under the unfixed plastic at this end and leave yourself a foot or two hanging free or tied off onto the locking rod. You will be tugging on this soon!
Now place the cleaned gasket onto the door over the top of the string taking care that the string sits in the step without being pinched under the rubber. Now lubricate the rubber on its outside faces with silicone spray.
Place the aly door mount onto the door and press it on firmly so that it will hold itself in place but still requires fully fitting. You will note that due to its shape and that of the locker door, you will not be pushing it on square but at a bit of an angle.
On the outside face of the door begining at one end, start teasing the rubber up into the aly door mount. Be sure to keep checking that the aly door mount remains firm along its entire length as you do this. I used a shatterproof ruler to assit with this as unless you are tough enough, your fingernails will be killing you before the job is done!
Now this next bit takes a lot of care and if you get it wrong you will have to pull the aly door mount right off and reset the rubber. Turn your attention to the inside face of the door - again tease the rubber gasket into the aly door mount but be very careful not to let the rubber slip into the step. To help you avoid this, leave the rubber showing outside the ally mount by about 2mm. Once achieved along the entire length or if you are careful working as you go, press the aly door mount further onto the door. Only now can you tuck the last bit of rubber into the aly door mount.
The aly door mount will appear almost fully on now but the rubber will likely not be fully seated correctly. To be sure it is, we now pull the string out from underneath the rubber along the length of the door. This is quite a hard pull and if your string is not up to it could easily break. As you pull the string out, keep firm pressure on the aly door mount, even hitting it with the palm of your hand to be sure it is knocked fully home as you go.
The door is technically now ready to re-fit back onto the caravan. I took the view that since the door fell out in the first place, it could happen again. So I added screw fixings to the inside face of the aly door mount using a 2.5mm pilot drill and No8 pan head x 8mm long stainless steel screws x 5no. If you do this, take care to mount them low enough so they don't snag the rest of the hinge mount fixed to the caravan (see last photo that shows these screws fitted - you might want to omit the end screws as these do pinch on the door seal a little which may cause wear in time, it closes fine though without obvious strain).
I hope the above makes some sense and so is of help. I am cetainly happy to have saved a potential £285!
llrides & Copper - thanks for your inputs.