It drops to Neutral due to sudden pressure loss (to avoid you being stuck in gear unable to move the car).
It reports "pump relay stuck" because the pump is constantly running, but the cause of that is usually the failed accumulator.
Saying that, a new pump relay at the same time is a good idea.
David is spot on. Just be aware, with the accumulator requiring pressure to be constantly pumped up the brushes in the pump may need replacing too, or soon. Just make sure the pump runs when you open the drivers door first thing in the morning and after you replace the accumulator the pump should also run every 2 - 3 gear changes. You can test this with the engine off so you can hear the whir of the pump.
Got the car fixed today after no garage would want to do the work. Even after I told them what I wanted changing. So I got the parts and I'm no mechanic but it was really quite simple and I had to ruin my 27mm spanner.
So change the accumulater and got drenched in oil ? It's under pressure. Then it would go in to gear so change the relay still wouldn't go into gear then I remembered that I brought oil so topped it up and no it's driving fine.
Go me! Thanks for the heads up guys much appreciated.