I have accidentally damaged the battery on my electric bike by not using it and not charging it for over a year. Does anyone know where I can get a replacement one at a reasonable price, I have been quoted 300 pounds or if I can get it repaired.

Take it apart to see if there isn't just a pile of size-D batteries all in series. If so, why not just go and buy the same number of rechargable batteries sold near the checkout at the market? It might take some skill to fabricate them together when the old ones I'm sure have spot-welded tabs to keep them together.

What kind of battery?

this is my suplier in china seems the cheapest I can find https://www.bmsbattery.com/theres nothing as good as LiFepo4 battery even are more economical with how long they last, probably yours was an acid battery they if left uncharged will die.

I took my Master E-Bike battery pack apart it was easy and it had 3 batteries in it that just unclip. I looked up the batteries and they were available for about $32 each then just plug the new ones in.