Plusnet refused to pay cashback

[#1]

I joined Plusnet recently via Quidco, who had a decent offer available for new joiners.

It all tracked OK (I'm pretty careful with these things!) but then popped up as declined.  I asked Quidco to look into it, and apparently Plusnet have refused payment because I used a discount code.

I don't see how that can be true - do you even offer discount codes?  I just clicked through the Quidco branded pages on the Plusnet website and picked the fastest broadband package on offer?

[#2]

Hi @jim1999,

I'm sorry for the confusion regarding your cashback and for the delay in getting back to you on here.

Having checked your account, I can confirm that the offer that you signed up for included £75 cashback. However, the cashback included within this offer is administered through us rather than Quidco which is why it will be showing as "declined" on Quidco's side. Your cashback reward was claimed on the 31st of January and you should have received the cheque within 30 days of this date.

[#3]

Thanks @EmilyD for the response.  That is pretty confusing. 

At the time Plusnet was advertising a £75 cashback deal on mainstream media outside of Quidco.

Quidco were offering ~ £104.

How can you have declined a cashback payment of £104 through Quidco on the basis that you had a separately advertised promotion for £75?  That doesn't make any sense.