Roll in a straight line still sounds a bit suspicious. What do you have your front tracking set to? And have you double checked it using a long stick to measure? Wouldn't trust a garage to set it.
On entering corners, where roll is inevitable, increasing front damper stiffness can go a long way towards reducing roll rate, with rate being the bit you notice. Increasing roll stiffness too much can cause the inside wheel to lift. Having said that, you'll likely have a 10mm anti-roll bar. Rough road cars and vans were fitted with 12mm anti-roll bars but these can be tricky to find in the UK. It is possible to connect 2 anti-roll bars together, and that's a common development trick to reduce the need for ordering parts. But that's all for corners, not for straight line.
If you want to knock heads together Clementine used to work for Lotus (not in handling, but I did share their office) and offers a £15/hr consultancy service. You could make a sticker to put on the side saying "Handling by Clementine". Fell free to pop down - I've got a tracking device too.