Fabia 05 1.2ltr Vibration Issue - Any Thoughts, Please?

Hi all, newbie here! I have an 05 plate 1.2ltr Fabia, just done 90k miles, and I've noticed the last couple of months, when I get to 40mph and put her into 4th, there is a vibration.

 I took her to the garage and they recommended firstly getting the front wheels rebalanced. I did this, but it's not made any difference (the guy who rebalanced them said they were only out by a small margin and he normally wouldn't have bothered adjusting them).

 I've been on a motorway run today, and notice that once the vibration starts at 40mph, it continues, but only when I'm pressing on the accelerator. If I ease off, the vibration eases, too. If I press harder on the accelerator the vibration seems to be more pronounced.

 I'm just wondering if any of you might have any idea what it could be?? (Obviously another trip to the garage is on the list...)

 Many thanks!