Year 12s: what will be the first way you research your future options? >>

Sports Direct changed price of my order and I did not qualify for discount voucher

TL;DR version:
Sports Direct promised me £10 discount if I spent £50 online which I did but my invoice is showing £49.99 so they refused me £10 discount because they changed price to 1p less after I paid.

Almost 2 weeks ago for May Bank Holiday, I went to Sports Direct website and in their homepage they were offering £10 discount if I spent £50 on products. So after an hour of browsing for products, I added £50 worth of products including one of their large mugs for £1. When I checked out, the total was still £50 and no £10 discount was applied. So I assumed they may email or send a £10 voucher code with my order. The delivery was £4.99 to home or in store. Quite weird as most other stores I've shopped never charge for in store delivery. If I chose in store delivery then I would receive £5 voucher. As my local Sports Direct isn't far, I chose in store delivery. I paid in total £54.99 including delivery.

Almost a week later my order was ready to collect. The woman who served me at the till gave me the usual £5 voucher but no mention of £10 voucher. When I asked her, she said I need to contact their customer services. I got home and opened my parcel and everything was good but there no sign of £10 discount voucher.

So I contacted the customer services and the woman who I spoke to struggled to understand me even after explaining to her 3 times. She said my order didn't qualify as I had to spend £50 not including delivery. I'm like WTF? Dude I paid extra £4.99 for delivery. I tried explaining to her again but it took a few tries for her to understand me and then said the same thing that my order didn't qualify because I did not spend £50. I eventually gave up as she was of no help so I put the phone down hoping to call again later and speak to some one else who is more sensible.

Meanwhile I checked my invoice and guess what. They have actually charged me 1p less the sly bastards. The £1 mug I added onto my order was is showing as 99p. So the invoice total is showing £54.98 which is why my order may have not qualified for £10 discount.

I have screenshots of my online order which showed £54.99 total spend.

Where do I legally stand on this.