I'm having a very similar experience. Got the same email yesterday. Went to my box to activate per the email but decided to call just before I did as the screen showed that I was agreeing to an additional £18 per month. I wanted to check that the credit indicated in the email would be applied afterwards. Then the current saga began...
Spoke to a lady who didn't know what I was talking about. She went off to check... She then came back and said there was nothing on her system about this offer and started to upsell for a further 18 month deal - not a bad deal to be fair! I explained that I wasn't on for a new deal but just seeking confirmation that my account would be credited with the £18 per month after I activated. She went off to check... She came back to confirm that as a regular viewer of Eurosport I was indeed eligible for a credit. She then said she could add a credit of £17 per month if I gave her the go ahead. I said I didn't want to do that because the offer is full credit of £18 and I couldn't understand why she wasn't able to confirm that. She seemed a bit confused why I wouldn't want a credit of £17. She said her system wouldn't allow her to give me an £18 credit!
On the basis that she confirmed that as a regular viewer I was eligible for a credit I went ahead and activated TNT Sports.
I received an email this morning confirming my new service and the additional charge of £18 per month was being applied as well as a portion for this month.
I called again this morning and spoke to someone on customer retention (I wasn't looking to cancel but was the only option on that painful phone system) to seek confirmation that the credit would be getting applied. He went off to check... He came back and said he had no information on the Europort/TNT credit on his system but said that credits can take up to 14 days to be applied. He then urged me to use the Live Chat as I could send screen grabs of the email. I tried that to no avail and was eventually pushed to WhatsApp. Conversation was engaged (at the time of typing) 40 mins ago with no response yet.
It has all the hallmarks of a scam but for the fact that the lady I originally spoke to did say that as a regular viewer of Eurosport I was eligible for a credit. A real standout is that neither of the customer service people I spoke to were aware of the email or anything showing on their systems, which is concerning.
Will be keen to hear about any solutions or responses that come up.