When I was on the shop floor many years ago in uniform and then plained clothed the script went like this;
'under no circumstances stop anyone unless you are 110% accurate that they have got an unpaid item and that you have had eyes on them for 110% of the time from them conceiling the item to them passing the last point of sale and intending to leave the store!'
All because if a case happened similar to yours, the guard/detective could end up in a litigation case of wrongful arrest, Assault, public humiliation etc.
If it was someone else giving you the tip off and not yourself that had seen the apparent theft then you should ask the accusing witness to accompany you to make the stop and also get them to make a statement. If they refuse and dont want to be a part of it then you should leave it alone and just keep eyes on the suspect and pass their details on the NET once they leave the store.
Also following on from above, when you do stop someone you are meant to follow a procedure of introduction telling the suspected of who you are and why you have stopped them, then, you should ask them to accompany you to an office at the back of the store, out of the way of public and unwanted viewing. You should also ask for the manager or senior supervisor etc etc of the store to attend the questioning/interview and always have a witness, even more so if you are male and they are female or vice versa.
They should tell them that you are wanting to search their bags and belongings and do they have any problems with this if they do then inform them that the police will be called to legally carry out a forced search. If they accept your request then make sure again everything is logged and witnessed. Also if you can, get the suspect to open and rmove the contents of the bags especially if it is personal belongings.
Looking at what you have said, they look like they have broken several rules, which if they have then it should all be caught on the CCTV by the entry/exit point where your mother was stopped. Also the store have 'if what you say is accurate' admitted liability by saying it should never of happened, however unless you have that in writing it means squat, as it is just hear say and they would deny saying it at a later date.
You may have a case you may not, it all depends on the accuracy and intricacy of your statement and the CCTV!
The best thing to do would write down everything said between yourselves and the store/security; times, dates, staff descriptions etc and approach the citizens advice bureau.
Incidentally before you left did you make a compaint to management or take any names or badge numbers etc etc.
A stab in the dark but is this store T"S"O by any chance? Just wondered as they have a history of this.
Hoped this helped..