When i first joined, i did so as an apprentice electrician and i think i signed something (it is 9 years ago), but when i qualified as an electrician i think i was supposed to 're-sign' to the company as my training had technically finished and i was a qualified electrician, but i never have.
Also, ive been office based now as an estimator for the last 3/4 years but still get paid and hourly rate as i was when on site.
In answer to the question, the agreed has always been as you said, 20 days (5 of those used for Christmas period), and 8 bank holidays.