2.5na wont start now worked other day

Me and my dad have just replaced his goosed 19j with a decent one due to using too much oil and smoking. Once the engine was put in it started really good we went for a short spin cup of tea then back to park it again. But when i brought it into the garage and turned it off my dad saw a spark from underneath. Next day my dad went to start it and it was like the battery was flat so charged it up still the same new battery heavy duty £85 still the same. Could it be the starter selenoid or possibly a bad earth.

Its really getting to be annoying

Check that the connections are all done properly - a spark would suggest that there was a live contact with the body/chassis somewhere, could your starter LIVE wire have been damaged somewhere - If it has say melted somewhere it could have a nice air gap now with some melted ends. Check your are getting 12V at the LIVE on the starter first - but don't rule this out yet - it could still be damaged/making bad contact somewhere. Are the battery terminals getting hot? That could indicate a poor contact there. Check that your earths are properly earthed - I went away for 2 months and when I got back it would not start - I took all my connections off from the starter, gearbox and chassis and filed all the contact points - put it together and it started straight away.

It could be your starter solenoid but I doubt you would have seen a spark like that if it failed - once your SURE your wiring is OK I would take the starter out and take the end off - the ends of the rods on the inside could be fried - it is possible just to turn them round.