We were caught in a compromising position in the stationery cupboard. Is this a sackable offence?

While you are entitled to a private life at work, this does have limits, and an employer has a right to expect some standards of behaviour when you're on company time and on the premises. An employer could well view this as gross misconduct, and a reason to dismiss you immediately.

If you are summoned to a disciplinary meeting about this, make sure to discuss your options with your union rep if you have one. They are entitled to accompany you, and can plead your case.