Accident waiting to happen

I have just returned from a great week at Incleborough fields CC and once again nearly come to grief on the way home at the same junction and do so every year.The road from Boston to Sleaford.As you bypass Sleaford there is a dual carriageway which is about 3 miles long and ends at a roundabout.There are 4 exits and the road clearly states left lane first exit Sleaford,second exit Peterborough ,right lane third exit A17 Newark ,forth exit Lincoln.I get into the right hand lane as I wish to go towards Nerwark and as i go onto the roundabout some comic always goes into the left lane and cuts across me.I have even had motorists going mad with me as if i am wrong.Im sure we have one of 2 types of motorist here.One who dosent want to be behind a caravan so they do it on a purpose not worrying if they kill someone in the process and the other that dosent seem to realise they are in the wrong lane.The road is printed several times with which lane to be in.Maybe the problem is that the white line that seperates the lanes dosent appear until about ten yards onto the roundabout but if you are in the left lane it becomes clear very quickly that you are on the wrong side of the white line if you wish to go to Newark.It frustrates the hell out of me as im sure there is going to be a accident.I have bought a dash cam but that wont help me prevent an accident.Is anyone else aware of this?