I've used the Smart Hub, TP Link W9970 and Openreach Huawei HG612 before. I'm currently using the HG612 and it's the most stable I've seen my line. Maybe pick one of those up cheap off eBay; I got an unlocked one for £20 but you don't need it unlocked, so you could find one for under a tenner. Or you may need the ECI B-Focus modem depending on your cabinet. If you don't already know which cabinet you're on, go to http://www.kitz.co.u.../btwchecker.php
and type in your landline number to find the exact cabinet number. Then go to https://www.telecom-...uk/codelook.htm
and type that number in the Partial Telephone Number to Lookup box and hit the Lookup Number button. A box will come up with the start of your landline number, your town and county etc... click on the town name. On the next page that comes up scroll down to the yellow box just above the map and click All xxx Fibre Cabinets. Scroll all the way down until you find your cabinet number in the list, and then look right on the same row to find the cabinet make. It'll say either Huawei or ECI.
If it's Huawei try the Huawei HG612 Openreach modem. If it's ECI then try the ECI B-Focus Openreach modem. They're both the official modems that used to be supplied before modem/router Hubs started being dished out, so if you do want to try another modem you can't go wrong with one of those. That is, if the issue isn't resolved and you need something to try
I've never had interleaving even with multiple modem reboots over a short period sometimes on BT. Maybe the modem doesn't work too well with it? I'm not really sure about the ins and outs of interleaving.