Thanks everyone for your replies. Some interesting points.
I didn't realise that by law an employer is obliged to offer part-time - my understanding is that they only have to give it serious consideration. I'd be interested to know under what conditions an employer would have to agree to part-time hours.
Certainly the cost of medicals and sim checks is an issue, but surely an employer could not argue that one if the pilots are responsible for the cost of their own medicals (and possibly sim checks)? As far as type-ratings go, what if many of the pilots joining the company are already type-rated and/or contract pilots?
Certainly that presents fewer reasons for an employer to refuse part-time? I suppose rostering could prove some difficulties, but it obviously works in some companies.
Reason for my original question is that it's something I may want to consider in the future but it's unheard of in the company I'm hoping to join and it's not something you can bring up too easily at an interview!
Any other points or info would be welcome.