Oh, and just to add, I work a 4 minute drive from my home, and the supermarket is about the same distance in the opposite direction, so it's not like I was far from home - I couldn't have been incase I needed to get back to the loo
So you were at a petrol station? So what? As a previous poster said you have been signed off as unfit to work, not as unfit to leave the house!
Are you a member of a union? If not speak to ACAS and see if they can point you in the direction of some leaflets etc. You should have the option to bring a friend or trusted colleague with you for support - useful if they can take notes etc for you so you can concentrate on what is going on.
Would it help to write everything down so you have some notes to refer to in the meeting?
Point out that you were less than 24 hours clear of diarrhoea. Explain that you have been feeling really stressed and worried about this and you feel that there is a malicious aspect to this allegation. If they get arsey, ask them to clarify exactly what you are supposed to have done wrong and can they show you the bit of the sickness management policy that you have breached.
Unfit to WORK does not mean unfit to leave the house. It is none of their business where you were or what you were doing as long as you weren't working at the time.
I had a similar situation, signed off work with depression caused by work, GP told me I had to get out and about as locking myself away would exacerbate the situation. Someone told my boss I was out all night on my birthday. I wasn't, I was in, but even if I had been it is none of their business and they have no right to ask.
Speak to ACAS, if you have a fit note that states you are unfit for work that's
Just because you're signed off sick from work doesn't mean you have to be locked away at home for that period. I never fail to be amazed at people who gossip about others being seen 'in town' or whatever. Different if someone's claiming to have a broken leg and they're seen out running but come on, in your case someone is being a right wally and you should insist in an apology.
You have been invited to an investigatory meeting took put your facts across and investigate the allegation.
You can prove your doctors appointment by asking the gp and by receipts in the supermarket, sheesh you can even tell them all about the programme you watched whilst allegedly playing volley ball whilst off sick!
It's down to them to provide proof that they know for a fact you are lying.
You went to the go who gave you medication what more is there to say?
You can ask but it's unlikely they'd tell you. It's to stop you going and decking them one before the meeting or intimidating them into withdrawing their accusation. I know your accusation is rubbish, but the employer won't know that yet and there will be cases where someone has been seen somewhere they shouldn't have been.
Do you have an appointment card for the GP (unlikely) or your receipt when you paid for your prescription, or the medicine pack / bottle (which will have the date on)?
They have to prove you were somewhere unsuited to you being ill - the shopping centre or the beach or something. Tesco's won't be unexpected because you still need to eat.