If they go ahead & transfer ownership, re road tax & do the admin you will have to stump up more than £100.

There should be some written terms and conditions relating to the deposit, which you should have been given, and which you can get from Arnold Clark if you weren't.

I've paid a £100 deposit on a pre-reg car with Arnold Clark. It is my understanding that, right up until the point of taking delivery of the vehicle, i can "pull out" and have my £100 deposit refunded - is this correct?

What would be purpose of deposit if you (or they) could "pull out" up to delivery?

As stated, their T&C's were surely provided at when deposit was paid and if lost will be easliy available.