Mondeo Edge 09 steering / pulling to side

Hey guys,

I have a Mondeo edge 09 2L manual with 45K milage. It was in a perfectly good condition when I bought it 9 months ago. I changed front tyres 2 months ago since they were wearing thin (new ones are Goodyear 215/55 R16 93W, rear tyres still Hifly 215/55 R16 97W XL). I also got a 4-wheel alignment. Since then began experiencing sudden swerving of the car to either side when driving on uneven roads, bumps, ruts.. etc. It's more noticeable around the back and on high speeds (40 MPH+). It's usually accompanied by some change in steering wheel direction. Done some research and I guess it's called tramlining but again not sure. Also not sure if it were there before I change the tyres cause I bought it in summer when roads were even, changed them in winter around the time roads became horrible. I checked it in a local garage which found nothing wrong with the suspension. Went back to the guy who fitted the new tyres to check alignment and suspension, said alignment is fine but the rearside offside control/trailing arm bushing is "wornish" and it might be the cause, would cost £90 and recommended doing it on both sides. He didn't seem very sure though, so I thought better than just do it.

Now I'm at loss. I don't know what this is or where I should go to get this fixed! Any help or recommendations? I'm in Nottingham by the way. Thank you.