Jump Start Puzzle - mss1tw
Battery on parents car (Proton 1.5 'thing') died overnight, it's all sorted now, bought and fitted a new one for them.

But trying to start it from my car using jump leads still resulted in it turning over slowly and not firing.

This was with the battery out on charge, with jump leads from my car directly onto the Proton's battery leads.

When a new battery was fitted it fired up first time.

Why would it not start when connected to my car? (Tried off and engine running)
Jump Start Puzzle - SlidingPillar
Either your jump leads are not up to it, or the connectors weren't. Even with good ones, I'd not expect them to supply all the current needed for starting, and always run the donor vehicle's engine for a couple of minutes to put something in the defective battery.

Not a good idea to try what you did though, as if the engine had started, and your leads had fallen off, the alternator in it could have fried.
Jump Start Puzzle - Simon
You need a decent set of jump leads to do what you did. Normally the jump leads assist the flat battery but what you were attempting meant that the leads had to carry all of the power. Most of the cheap jump leads that you can buy off the shelf are hardly up to the job and the longer the leads, the better they need to be. Think how thick a normal car battery cable is and compare it to your jump leads. I'll bet that the car battery lead is probably twice as thick as your jump leads.
Jump Start Puzzle - mss1tw
Cheers guys - makes sense now.
Jump Start Puzzle - David Horn
Bet your jumper cables were warm to the touch with 500A going down them!!
Jump Start Puzzle - mss1tw
:^D Yes they were defintely more pliable afterwards.
Jump Start Puzzle - L'escargot
Cheap jump leads have a high resistance and consequently there is a big voltage drop from one end to the other.
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L\'escargot.
Jump Start Puzzle - Vin {P}
I had a pair of cheap jump leads years ago that were OK for occasional use on my bike. When I got a Granada 2.8 - oh, the joy - I used them and the insulation melted in a haze of smoke.

Bought some decent ones and they have lasted till now (15 years)

V