never trust any jack if your going under ,good stands are a must , i have a halfords one it works ok but its not like a pro one , i got it on a deal too.
I have actually got a big monster 'industrial' jack sitting in the garage - one of the big orange ones the garages use with a 4 foot long permanent handle which was given to me but does not work. I assume it does not work because the oil has leaked out and I assume the oil has leaked out because a seal has failed - may be better off getting that working - do you think i'd still be able to get the seal.
This^^ I'd get the garage one working too... you'll probably find that it's the seal on the pump piston that has gone... it's what happened on my last jack!!
I have a big one I got it from a local tire fitting place they had got new ones i changed the oil in it and its been great
I have one I bought from Aldi a few years back used with a block of wood still works great. Make sure your axle stands have pads on the feet otherwise you end up with lots of little triangular holes in the driveway as they sink in. No I do not have any triangular shaped holes but I know a man that has.