Taz, I used to work for a Bloody Awful airline along the Bath road. I had a B707 Type Rated Licence but was still a Tradesman. Our Supervisor was away on leave and the company thought it would save a few pence by not making anyone up as Acting in his abscence. As luck would have it, they needed a Certificate of Maintenance signed on a B707 and I was the only licence holder on duty. I dug in my heels and refused to sign unless they paid me the Acting allowance for the full period of our supervisor's leave. They coughed up and I signed, no come backs.
The moral is, No Money - No Honey!! If you want to avoid being walked over, stand up for your rights and call their bluff lad. That's what Balls are for
P.S. I was promoted too, just a coupla months later.
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Info noted. Plse report further.