Help needed please.

Brief History:
Owned car for 10years had Mi16 conversion done about 7 years ago by Pug Performance. Car ran fine for year (16,000 miles) or so then bang! Re-built engine with a friend but managed to get dirt on the crank which meant it had to be re-built again. This was 5 years ago and in that time it has only done another 250 road miles.

New crank/bearings/starter/lifters etc. when re-built now the problem:
This has occured approx 3 months after the engine was rebuilt but I didn't have time to fix it, so it has sat in my garage ever since waiting to be looked at, (I keep buying other cars and putting it off).

When the battery is fully charged the car will start but the starter turns quite slow, turn off, start again, bit better this time, turn off, starts but turning slow, turn off, fails to start, starter turns but very very slow, try again and nothing just a clicking sound from the starter trying to engage. Battery then goes flat and the process will start again once the battery has been on a charger for 10mins.

Took the starter motor back and had it tested - o.k so what next?
I also spoke to the head of pug rallying at Ryton and he mentioned that the starter could still be at fault, or new battery but the battery was new when it started doing this 5 years ago. I,ve no idea now and having got the car near concours standard I,m desperate to get it running again.

Any help gladly welcome. (p.s i'm away this weekend so don't worry if I don't reply)