In my Opinion ,
Your contractual rights to Interest on Back pay can only be determined by your individual employment contract (which is probably completely silent on the matter). Custom & Practice and the application of internal processes to your colleagues' T&C's may be relevant to your claim under the Equal Pay Act.
I suggest the following given your fairly recent tenure with your employers
If you have a trade union or staff association body raise the issue on a generalistic and non -identifiable basis through them.
In the absence of the above and your presumed requirement to retain your current employment tread carefully.
You may wish to keep bated breath until at least you have enjoyed/endured one year's employment with the Company.
At that point you wil enjoy certain legal protection in relation to your current employment and your employer will or should be expected to be aware of this.
This should allow you (if you so desire at that time) to put in a backdated claim with minimal chance of receiving the ultimate sanction. In the real world, I would recommend you undertake a mental cost/benefit analysis of taking this step.
This is only an opinion given by an individual who asserts no legal background or qualifications but seeks to assists others throught the exercise of opinons gained through life experience.
Please Read,Interpret and Action entirely at your own risk.
Finally, Good Luck