Hi everyone thanks for the all the replies so far, we loaded a 100ft spool (with the black discs) into a AATON LTR mag that has the 100ft 'key' on the feed side, the film ended up on a typical 16mm core once exposed.
The load was done in what we believe is a light tight changing bag, but it is a no brand one so we are now slightly dubious...
One crucial piece of info I missed is that we accidentally taped the mag up too close to the gate lock in area, which meant we ran an entire false take of the stepped zoom sequence where the film was not actually traveling through the gate, (it must have been a fraction of a mm away and therefore not taken up and through) I suspect that the film was unraveling on the feed side at this point.
We corrected our taping and loaded the mag and started the test - we did not open the mag.
Then just after the first exterior shot the film jammed, opening the mag in the black out bag revealed the film had seemed to had spread out all over the mag on the take up side, some had rolled on to the core, but then it had gone a wry, ribboning and waving all over the mag.. we feed it back on the take up core and all seemed ok. Then we did the final exterior shot with the skates..
I think our black out bag is to blame...