Employee or consultant - whats easier/more economical

I run my own CC and have been offered a part time position at a previous company I have worked for. Would/should I let my employer pay on my behalf, or would it be better to invoice them as a consultant and pay my own tax. During the time I will be employed I will also be invoicing out other clients through my CC.

What will you be doing there, how many hours will you be working?

Consulting on systems/processes and training - approx 100 hours a week

Given that amount of work performed and I assume it will be on their premises and under their supervision you would be considered an employee of the company and therefore they have to withhold PAYE each month.