Incorrect referral to credit management company

Hi sorry if this is similar to previous threads.

Spark gave me an incorrect bill last Oct when I switched providers and it took months to sort out due to issues with their software. Meanwhile they referred me to a credit management company. I refused to pay and finally my new provider managed to get through to Spark and sort it out but I never received a final bill from Spark. 11 months later I get a message from another credit management agency saying I need to contact them. Turns out Spark has finally issued the correct bill but I was never actually billed it. I haven't received a paper or email bill but the correct bill is showing when I log in online. The bill was issued in the end of August 2017. So I've only just missed the deadline (3rd Sept) but I've received no bill or warning of missed payment.

I'm just wondering what I can do. I'm looking to buy a house with my partner and I'm super paranoid that this will impact by credit score. I'm going to ring spark up to pay the bill but I want to make sure that this doesn't go on my credit record.

Any advice would be greatly appreciated!!

Thanks in advance.