Thank you everyone, no i don't know exactly what he died of but i'm pretty sure it was a brian problem.
When i think back to the past, he had seemed sad recently and I'd even mentioned it once or twice. I never thought he was ill though because he was eating and pooing fine so i assumed everything was ok. He always jumped up as soon as i got out food and seemed his normal self. He didn't seem ill, just sad but he probably had a headache, i wish i'd known. They hide illnesses so well.
We'll speak to lynne (person caring for him when we were away) and the vet but i don't want a pm done.
Jasper was bonded with tink, mischief and bloss but after the 2 girls fought, i made the hard decision to split them into 2 pairs. Jasper went with tink because she was the one he stuck with the most. So she was with him when he passed away, she sat with him and groomed him when he'd gone.
After a good 20 mins, lynne took jasper to the vet to look for obvious symptoms as to why he'd fitted and died. Nothing obvious and all the other rabbits are fine so we're 100% sure it wasn't E.C. Tinkerbell is still grieving but poor mischief and bloss won't know where he's gone.
I'm not sure whether to show jasper to them before we bury him but my mum said he won't smell the same and they won't know it's him.
I'm also considering putting tink back with mischief (they're brother and sister) and then sort something out for bloss... but she loves mischief too.
I could try a trio but i was told by the vet that the girls can't go together.
But now there's only 3 buns, will the dinamics be different and might they get along? If not I'll have to consider a new bun but I don't want to do that anytime soon. It wouldn't feel right.