Scooby Toy

Hi - Brand new to this site and looking for help already.
Son bought a '95 STi a while ago, but its now costing (me) a fortune.
Head gasket blew so local "specialist" stripped the engine down, had both heads skimmed and tested then rebuilt with new timing belt and all the pulleys at a cost of £1,900 - I supplied the metal layer gaskets and high torque bolts.
Car ran for around 100 miles then the bottom end ran (sounded like a bag of spanners). Local guy quoted another £2 grand to replace the engine with a reconditioned unit, but I opted to do it myself - possible mistake.
Fitted the new timing belt, pulleys and bearing from the "old" engine to a reconditioned unit from MR Engines (£800) but when car started it idled at 3,000 rpm, pulled the vac hose off from under the dump valve and it dropped back to 1,000 rpm.
Stripped out dump valve and cleaned out some crud, rebuilt and the car started up and idled as it should. Car has been sitting for 2 weeks and now won't start, it offers but then backfires.
Any ideas on what it could be? - these things are b****y complicated - old engine management system was tweeked to the max and ran at around 370 BHP.
Any suggestions will be gratefully received, this car is driving me nuts.
Begining to wonder if the timing belt has jumped ... ... !!!