BT/Lowell old debt

Hi, I just wanted some advice.

I received a letter from Lowell regarding a very old debt - We had a BT account in 2007-2010 which we closed when we moved house in December 2010. We then started with Plus Net.

Since then I've periodically been sent dca letters from lowell trying to collect £60-£70. I disputed this in 2011, saying that these are charges that I had already paid, and sent letters saying that this should be let go. BT never replied, we moved house again in 2015 and in 2017 I received a letter again asking for this money. I assumed that as it was so long ago that soon it would be stature barred and I could forget about it anyway, regardless. I got another letter last week, so I checked my Clearscore account and it's not there. Someone suggested I have multiple checks to see if there's anything lurking and the lowell account is showing on a totallymoney credit check....and it's showing as having missed payments every month for the past 8 years!!! No wonder my score/report is so bad. The account says July 2013 as opened with lowell....now I'm unsure about what to do. Options as I see it:

A. Continue to ignore and see if it disappears in July 2019.
B. Send letters asking for them to prove the debt etc and try to stop the damage.
C. Take Lowell up on their F&F offered in their letter of a 60% discount (£30 of £68), and say goodbye to it forever.

I'm really sore about paying it - I don't owe that money, I had such a horrid time with BT and it was such a long time ago, I don't have the bills etc anymore - it was nearly 10 years ago and one a different bank account too as we switched in 2012.. I would dearly love it to go away now, one way or the other.

What would people recommend?