Yes, it might be just a super-slow first drop but its starting to look pretty weird. Let me just think out loud again...
So you've got this new 63L/17G tank with built in filter. We've been over the media, right? Nothing strange, just biomedia like sponge and/or ceramics and filter floss or pads? You've had it on all along, right? There's nothing in the tank except some sand or gravel, right? We already know your pH and temp have been just fine. The boots ammonia did not foam when shaken, right, and its plain and clear? You filled the tank with tapwater, right? You haven't added any other chemicals or objects other than treating the water with stresscoat conditioner, right? You haven't cleaned the filter or added unconditioned water, right? You're using a liquid test kit, right?
If we get to 21 days and it hasn't changed then I think we should do something. One thought would be to get a small bottle of prime and do a complete water change and use the Prime instead of stresscoat. I made this switch once during a fishless cycle based on what I remember as a few mildly negative comments about stresscoat, but I can't find the reference anymore. I take it the MM search in your Norfolk UK area never turned up any donors, I guess?
~~waterdrop~~