Where do I stand?

General_Grant

First you need to know what was originally in your written terms of employment.  The law says the principal statement must include "job title or a brief description of the job".  What does your existing document say?  Could the title (which is usually what organisations give) one which could be applied to what you currently do?  (And you've said you are still doing some of the tasks.)  Note that a full job description is not required.  It's meant to be just something which allows your position in the hierarchy to be known.

Does your original job description include something like "and any other duties reasonably requested"?

erb91

My prior role was to manage an area of sales in our store.

The contract as most do, states the company can amend duties from time to meet the needs of the business. However does doing the role of my previous manager with such an increase in responsibility being in charge of the whole store fall into reasonable additional duties from time to time, without this in written form? Especially when sold to me as a promotion with pay rise to match.

This is a promotion with failure to provide pay and official terms as promised which has lead me to want to step back from these additional duties just as the company stepped back from following through with their promise whilst I’m effectively doing a whole other senior job plus my old one for no additional pay.

lincroft1710

A pay rise upon promotion is not enshrined in law and a verbal promise of such is worth the paper it's written on. You will struggle to prove a pay rise was promised or was unconditionally promised. Also by doing the additional duties at the same rate of pay, you appear to have accepted the promotion.

Obviously it is up to you what you do now, but why not try to negotiate a pay rise and/or reduction of duties. 

robatwork

Doubtless your company will say that budgetary restrictions have meant while they appreciate staff being flexible with extra work and changing responsibilities, they didn't promise any pay rise. 

All you can do practically is keep badgering them, politely, for the rise. They sound like a "put up and shut up" kind of employer. Your realistic options are to do so, or leave.