Serious problem getting parts from Adria :(

It depends what parts are required. Sometimes you can obtain the part from ebay or the original manufacturer. We wanted a rear end light unit, six weeks from the dealer via the manufacturer, overnight on ebay. We wanted a drop down bed strut motor, six weeks from the dealer, within a week from the UK agent and a fifth of the price.

Ask the dealer for a copy of the paperwork for the order. It won't be the first time a dealer has forgotten to order the parts or ordered/were sent the wrong parts. You need to track was has happened so far and why.

I believe you have rights under the Consumer Rights Act. I've no idea how this could work for someone in Northern Ireland [sorry]:

https://www.citizensadvice.org.uk/consumer/somethings-gone-wrong-with-a-purchase/if-something-you-ordered-hasnt-been-delivered/

You might be stuck though if the part is essential to the repair. I would write to the dealer, copy to Adria UK, and refer to the Consumer Rights Act whilst asking for a full explanation of the delay and a fixed date for the part. If you prefer email, send it as a PDF attachment.

Your dealer should sort this out, not you. You say that they have been as helpful as possible but I'd say they haven't been because you have not received your part. 

If the MD is a male, send a letter in a pink envelope with just a hint of perfume, address it personally and you can bet nobody other than the boss is going to open it. For a female MD a dark coloured envelope and aftershave! It's a last resort but it usually works. 

Your contract is with the dealer and legally the fact that they are entirely reliant on Adria is irrelevant. Not sure which Act covers sales in your region but essentially the repair should be carried out within a reasonable time or you will be entitled to damages for this or you may even be able to reject the van. You should be able to get more detail from the clubs Legal Help Line which will have more information on your rights across there than I have.

Approaching the dealer on the basis that it may not be their fault but could be the best way but the threat of legal proceedings particularly in your region may work better than anything else.

was the damage done by yourself as you say you paid for the parts. which means you couldn't reject the van. would they take this long if it was a warranty repair ??. did you pay for the parts on a credit/debit card as I would give the card company a ring and claim money back and try another route for spares.