Pay advertised as xx, now I've started it's changed

[Bombaypotatoes]

I work PT-in interview and on the job advert it states pay is xx per hour but rises to xx (nearly £2 more per hour) once all training is complete. The higher amount is one of the main reasons I took the job as it was more than I was currently on and the hours we're more family friendly.
I've been there 5 months now, completed all training and the pay rise has only been 20p. So I'm earning less than I was in my previous job.
But so do I have a leg to stand on, can they legally describe pay as xx but pay me differently? Want some advice before I bring it up

£2 an hour more is a lot - what reason have they given for not raising your pay so far?

This seems very odd - did. You receive a contract of employment with pay detailed? Did they discuss pay in your interview? Do you have a copy of the advert?

What does your contract/offer letter say?

Are you sure the end of training rate was the £2 higher one? Usually, the ads state the whole payscale, which whilst in theory you could be paid any where in that range, usually you will gave been in the job for several years before reaching the upper end.

But have you brought this up with your employer?

We have kind like this in my place of work but the employees manager has to submit a report to endorse the fact that have completed all the appropriate training and are working and competent at the higher grade?

Pay and Finance won't automatically know if no one has told them?

I have asked and have been given various excuses by different supervisors. One said there is no pay rise, one said it kicks in after the new year, one said they were adding more training before anyone gets a pay rise, one said they didn't know what was going on. Big boss is away at the moment so can't ask them.
My contract doesn't mention pay but the letter of acceptance I signed mentions the lowest pay rate. But that doesn't explain the 20p pay rise.

Although different supervisors have given differing "versions of the truth', I think you need to focus on what your line manager / supervisor says. Otherwise it is creating unnecessary confusion and distraction.

Get your Line Manager to say what applies to you personally regarding your pay increase.

Mention you have been committed to attain the necessary competence for the pay increase, and ask them to confirm if you have reached the required level, if not what is the competency gap and how can you work to fill it and in what timescales. Ask for confirmation in writing and try to work towards a plan which includes all these elements so they don't shift the goal posts or deny their side of the bargain.