advice please

just under a year ago bought a year old ex rental moho from the rental people. all looked good, not that i knew too much about them being ex caravanner. August this year i noticed a small crack appear by a lounge window could only be seen when window open. obviously e mailed picture of same to people bought from saying it was obvious there had been a repair job going wrong. They checked records and said they had no record of any repairs but offered money towards cost of my repairing it as gesture of good will. when it had its habituation check the engineer agreed with me about it having been repaired and pointed out that the inside of the concerned window wall panels did not match the other panels. Also that the roof has a hollow dip in it and had obviously been damaged and repaired poorly.
The upside is that there is no damp evident on the repair sites, but obviously if i want to resell in future the value is going to be affected. has anyone experienced anything similar and got anywhere with the sellers, please.
thank you