I've pieced this together from a comment that Ribski made on here a few years ago about older Eriba door locks and from a recent question by Nigel Hicklin on Facebook, answered by Frantone.

The question by Nigel Hicklin:- Can anyone advise please on how to remove the door lock on my eriba familia 310
The mechanism is sticking and the catch wont slide back out again.

Ribski's comment:- (relevant to older vans) I've not actually removed the lock on my door, but looking at it, I think you need to remove the 4 blanking plugs on the INSIDE to gain access to screws, They are black, like the door lock, and are FLUSH with the black plastic cover on the inside. They may be difficult to see, and look like small circular 'mould lines' in each corner. If this is like yours they should be eased out with a small pointed tool to reveal the 4 screws :)
Hope this is right for your lock and that it helps :) As I said I've not done this myself, just my view on how I'd tackle mine if needed !

Frantone's reply and photograph:-

The coloured covers are over screws. Remove carefully (use a pin?) then the screws are revealed to open the lock. I believe WD40 can cause some nylon/plastic parts to swell so other lubricants should be used. Dry graphite spray for locks I think.

The Result:- (comments and photos by Nigel Hicklin) :-

Had a look at the door lock today and it turned out very easy to sort out.
Take the cover off the lock and the red and green plastic plugs out.it exposes 4 screws, remove the screws then the lock comes away reveling the catch.
Sprayed with wd40. Moved the handle on the outside a few times to work the wd into the moving parts.put back together job done only took 15mins.

( I think many Folk would not agree to the WD 40 bit).

.