All but the basic spec 340's even with a 1.4 got 175 wide tyres, it's listed in the brochures. The later cars badged just '340' got the skinnier 155 wide tyres and I believe DL's also. I had 185 wide GLT wheels on my old 343 and it didn't really make the steering much heavier than standard, the usual suspect is as Dai says - failing/worn top mounts.
35psi is the first thing to try I'd say - for a start it's free
