Mary Portas is very obviously munching her lunch during our (pre-arranged) phone interview and keeps breaking off to answer another call or give instructions to someone.

Perhaps I’ve caught her on a bad day or perhaps I’ve got up her nose just two minutes into our chat; whatever, she’s much less funny than she is on air.

I mention that I went to school in her home town of Watford, but instead of responding with something polite, all she wants to know is how long I’m going to take and why I keep asking her about how depressed British high streets can sharpen up their act.

The tall, notoriously well groomed red-head is just about to start a national tour promoting her memoir Shop Girl, which looks fondly back at her upbringing in a working-class Irish family in Watford and the influences that helped shape the woman we now know and love as the champion of independent shops.

And she should know; when the teenage Mary lost both parents within two years, it was she who had to shop and cook for her younger brother,  an experience she says “she could not have survived” without the help of the local butcher and grocer, who used to put things aside for her.

A somewhat unlikely but popular convent schoolgirl, Mary had to turn down her coveted place at RADA after her parents’ sudden deaths and sign up instead for a window dressing course at her local college.

Describing herself as “never knowingly under performed”, she says she found an outlet for her thwarted theatrical ambitions in creating highly dramatic window displays for Harrods and then Harvey Nichols, the Knightsbridge department store immortalised some 20 years later by Patsy in BBC TV’s Absolutely Fabulous.

Yet the might-have-been leading lady struggles to name a character she would have liked to play had she succeeded as an actress.

“I love giving live lectures [as a brand consultant], so it would have been something really exciting in the theatre, something with an accent, perhaps [Ibsen’s heroine] Hedda Gabler,” she says eventually.

These days, when she’s not running her own retail consultancy and looking after three children (two young adult children from her first marriage to former teacher Graham Portas and a toddler son with her current wife Melanie), Mary says her favourite form of relaxation is “getting out on a frosty day and getting warm in my walking boots and jacket. I have to be near open spaces to clear my head.”

Very refreshingly, she’s not bitter or even wistful about her missed opportunity on the boards: “I’ve had a wonderful life and I really think it’s my path” she says of her retail career.

And how would she like to be remembered?

“As the girl-done-good, as someone who did some good and as someone [who knew how to] lead with charm,” she says, without a moment’s hesitation.

Funny, no - at least not this morning (she later proved to be extremely amusing when I interviewed her on stage at a book event) – but definitely a woman who knows her own mind.

Mary Portas can currently be seen each Tuesday in Secret Shopper on Channel 4.

Shop Girl by Mary Portas is published by transworldbooks, price £16.99.