The pilaster strip on the top cupboard of my Eriba 420 have split and will no longer hold the hinge screws. I have contacted Hymer but they say it is out of warranty. Is there a DIY solution?

A few points.
It would be helpful to know how long you've owned the van Andrew, because you might have a valid claim against the dealer under CRA if the van was sold to you with the defect you mention.
Secondly, never be put off by a dealer saying the manufacturer won't accept a warranty claim. The warranty is just a method for the dealer getting paid for rectification work. If something is wrong it's the dealer's responsibility to sort it and whether he gets repaid by Hymer is of no concern to the customer. Too many dealers make it sound as if refusal to accept a warranty claim means the customer pays. It's just the dealer trying to wriggle out of their responsibilities.
If you've owned the van for long enough that the failure is not the dealership's responsibility, then have they quoted for a proper repair or have they, like some, refused to repair a van they have sold? If the latter contact Hymer direct as that is surely breaking the terms of their authorised dealership agreement.
Finally, as Colin suggests, some pics would help identify the problem and give appropriate advice.

Thank you for your interest, The pilaster strip is awkward to photograph but below are my best efforts/Users/andrew/Desktop/IMG_0469.jpeg/Users/andrew/Desktop/IMG_0468.jpeg/Users/andrew/Desktop/IMG_0473.jpeg

Thank you for the advice. I bought my Eriba 420 second hand from Automotive Leisure in Poole on 17/09/2016.I enclose some photos of the problem./Users/andrew/Desktop/IMG_0473.jpeg/Users/andrew/Desktop/IMG_0469.jpeg/Users/andrew/Desktop/IMG_0468.jpeg