I'd be interested in knowing if the engine itself was actually turning over. Bonnet up, someone in cabin, watch the engine when they release their foot from the brake?
It couldn't be the clutch unless you've noticed that the car doesn't move when you're driving it - a clutch that slips that badly wouldn't transmit much power into the gearbox. However, it could be the case that you just have the car towed everywhere and haven't told us - but if you're actually driving it and the clutch seems fine, it's not a slipping clutch.
First gear is 4.692:1 which is higher than reverse (4.260:1) but many complain that first is too tall for these cars. I can't disagree, I've driven a few manual D40s and they need a little clutch feathering to make them spritely. You could engage 4LO which would turn first gear into 12.3:1 - if the car can't hold on that, then either your compression is shot (that should be tested if in doubt) or you're parking on something that's really too steep to park on without wheel chocks.
On that latter point, what I used to do with my VW Kombi (crappy brakes, engine compression something like 2:1 and a total of 12.5 dpp (dead pony power, significantly lower than horse power) was park the car facing down the hill and point the steering wheels towards the gutter, then let the car roll forward until the gutter became a wheel chock. Suffice to say that I never had the car roll forward on me, but I always had fun and excitement trying to get it back up the hill (the car always ran slower in the morning due to the additional weight of the dew that had settled on it).
