Maybe one of these?

Left hand

http://www.caravanownersclub.co.uk/Shops/Leisureshopdirect/Product/664542/End-cap-for-side-runner-L-H-for-Seitz-Blinds

Right hand

http://www.caravanownersclub.co.uk/Shops/Leisureshopdirect/Product/664541/End-cap-for-side-runner-R-H-for-Seitz-Blinds

Great thanks Nev

This may help ? If you have most of the broken bits, why not try a good Super-glue (cyanoacrylate) ? There is an 'accelerator ' aerosol product available which cures the glue 'at once' so makes it possible to build up hollows and any small missing chips etc. This, of course, is subject to the type of plastic ! Probably a good idea to strengthen the hole with a small washer, as previously mentioned. I have used this method, using the accelerator , repairing plastic interior trim and broken facia securing brackets on various cars ! Just maybe worth a try if you can't source a replacement part !

Thanks Martin and Pete, quick is not a neccesity as it will be a week or so before use, so once fixed we can leave it. On the hunt for tiny washers in a minute.

"quick" is more a case of being able to 'build- up' any gaps in the joint, superglue is very 'liquid' and has little gap filling properties and may not make a perfect surface to surface fix ! By applying and 'fixing' at once you can build up the joint - I've found that sometimes when you try to build up normally cured joints that applying more glue seems to 'dissolve' the earlier applications - unless a very long time is allowed between the applications !

Good luck with your repair !

OK thanks again

First attempt which may be OK. Took the whole blind off, then tried to match the broken bit to where it should be. No good, my dodgy feelings at the end of fingers and shakes ruiined that idea.
Carole with her oh so delicate and tiny hands managed to get pretty near on a tryout, so I clamped the main part to prevent me wobbling it and with tweezers she got it fixed using a smear of Loctite Superglue just to hold it in place.
Later after lunch I aim to do the job as detailed above with a big blob of Araldite Rapid and a washer.
Hopefully it is possible to drill through the glue.

Have just noticed the other rolloer blind is slightly loose at the same spot and when tightening the screw, it's not biting, so a longer screw maybe necessary.

wider screw not longer don't want to puncture outside alloy bodywork

Good point although had thought about that.

All done - hopefully. Thicker screw too. What are they screwed into?

Next job replace a broken off side mudflap. Son or nephew to help once new one bought.