In all fairness to the bloke he has tried and if he hasn't charged you for the work he has done since the flush then he has done what he needs too.
You say that he has left you in a worse position but you also say that you have had a problem since you moved in 15 years ago so there is always the possibility of doing something and making the problem worse.
No he has come up with 2 options then you can make a choice between the 2 or get someone else in.
The rads may have been oversized for the pipe work but if the others don't work with the new rads turned off that is not the problem.
You say that he has left you in a worse position but you also say that you have had a problem since you moved in 15 years ago so there is always the possibility of doing something and making the problem worse.
No he has come up with 2 options then you can make a choice between the 2 or get someone else in.
The rads may have been oversized for the pipe work but if the others don't work with the new rads turned off that is not the problem.