Never soldering anything again. Made the connector, connected a battery, got a single shot off before nothing and the red LED.
Switched batteries, emptied the rest of the magazine (3 round burst is my new favourite thing), then as I've disconnected it the MR30 has come off my connector and got itself stuck inside the one on the MP5. Dug it out with a pair of pliers, ordered one off Eagle 6. Link below.
Also burnt myself with the soldering iron and dropped a scalpel onto my leg ?
https://eagle6.co.uk/shop/catalog/product/view/id/7874/s/eaglesix-female-mr30-to-male-deans-adaptor-8cm/