Cable hubs usually run hotter than an Openreach-connected VDSL hub because of the intensive computation required for DOCSIS. Normally that's manageable, the recent high (by UK standards) temperatures are possibly taking the hub above its operating temperature, but if that's the case then you should see a red LED on a Hub 3, and a purple ring on a Hub 4. If you're not seeing that, then potentially the technician's diagnosis is incorrect.
Interesting that this has just cropped up. What can you do about it? Not much if it is heat related. VM mention finding a good place for the hub, that's a laugh and a half when the company often install with beggar all excess cable, and then charge a hundred quid to move it or provide a longer cable. Maybe remember this when your contract is up for renewal. In the meanwhile keep it under review, consider putting the hub on a cheap laptop cooling pad, bearing in mind that those only circulate the air more, and the Hub 4 has a fan already built in.
The hubs should have been better designed to cope with a hot British summer, after all this isn't Arizona, but they've always been budget engineering, and there's a lot of things that the hubs should have been better designed for.
