Bongo will not rev when in drive

[ScaleyDog]

Hi everyone, I am new here. I have a 2004 2.0l petrol bongo. It has been sorn for about three months. It wouldn't start so paid a mechanic to come round but it still wouldn't start - suggested not firing at correct time (crankcase speed sensor or timing belt slipped). I checked the speed sensor worked (powered and ran a spanner in front - ok) and then checked timing belt not slipped - ok. Replaced plugs and it started. It revs ok whilst stationary but when in drive, you press the accelerator but it won't rev up. I lifted the rear axle and the wheels spin and get faster when revs increase so issue only under load. Any suggestions, please!!!!!
NOTE: I have a sloping drive and no parking spots locally so can't be moved as I can't get it back up the drive.