Parents were on the international3 since TalkTalk inception (about £30/month all in) but I was reluctant to change them onto a new contract before we knew what was going on with FTTx on our cabinet.
Once it became FTTC and talktalk were able to provide that I was happy to swap.
I wanted a medium fibre for £5 deal they had offered to those on low speed ADSL but it finished the week before I tried....
I pleaded a bit and got put through to retentions who set me up a great deal of medium fibre and anytime calls for £10.75 a month.
Whether it was length of time, or the £30/month or both I don't know, but the feeling I got was that we hadn't had the best value for those 6 years, and this new deal was able to be lower because of that.
So yes, definitely try retentions. Actually give them a figure you;d like to pay and see if they can do it
In theory, theory and practice are identical.....in practice they aren't!