M&S Diddling Customers Out of Cash?!

I recently bought some bras from M&S while they had their "Spend £30 on Lingerie, Get £5 Off" promotion.
I had to buy them online, as they rarely have my size in stock in store.
So, I must have ordered ~£150 in bras in total ( ) as I wanted to try a few of their different designs on, to see which fit the best.
There were a couple of bras that didn't fit very well, totalling £65, so I took them back. However, when I returned them, I didn't receive the full amount back. When I questioned why with the cashier, she said it was due to the items being bought under the "Spend £30 on Lingerie, Get £5 Off" promotion. I pointed out that actually, I still had ~£90 of lingerie items still listed on the receipt that I was keeping, so should surely be entitled to a refund of the full listed price, as the promotion was still valid across the items I was keeping (i.e. I'd spent over £30 on the remaining items, so should still be entitled to the £5 discount across these items). The cashier didn't have any reply for this, so advised I contact customer services, which I did.
Their response was that the discount was divided equally across all applicable items and so if any were returned, I'd be refunded the amount I'd paid for them (i.e. the discounted price). If I'm being picky, the receipt doesn't actually show this - it shows that I paid the full price, with the discount applied at the end, assuming that the £30 spend had been achieved.
And actually, the way they work it, by not giving a full refund, means that I only received £2.75 off a £90 spend on lingerie, not the advertised £5 off a £30 spend
This has now happened again on another order where I bought some vests & knickers & had to return some ill-fitting vests, meaning I've only received £3.70 off a £40 spend
I know it's not huge amounts of cash, but if M&S are doing this to all customers, then it'll soon add up... in their favour

I would send this in a letter to them and what they say once its in writing.

I would send this in a letter to them and what they say once its in writing.

i agree with this

I would send this in a letter to them and what they say once its in writing.

Great minds think alike
I wrote all this in an e-mail yesterday evening and sent it off to their customer services dept. (again) so at least it is in writing
Not sure how far I'll get though, as my original complaint was sent via e-mail (rather than telephone) so I could keep tabs on it

That's how Tesco do their discounts too it's most annoying if you need to return items

That is really poor customer service.

Hun, don't let it drop and keep kicking off until you get a refund. In cases like this they get so fed up that eventually they will give you a refund. Just keeping asking to speak to the person's supervisor and then their supervisor.

That's how Tesco do their discounts too it's most annoying if you need to return items

Boooh! Tesco do it too??
I was wishing I could still get hold of a copy of the T&Cs for this particular promotion, as I'm thinking if the refund policy isn't mentioned on it, by not giving a full refund/applying a full discount to the remaining items they're breaking the Trading Standard rules...?

That is very poor CS, I would expect more from M&S.