Check out
www.thetrainline.com
They only do tickets 3 months in advance so you can't yet book for May - the route looks ok - either change at Stevenage, or at Peterborough then Doncaster.
Should take 3 - 4 hours. Prices anywhere between £20 - £40 for a single. No idea if you want a return, or whether you will be going on to Hartlepool
Good luck in getting tickets
Dave