How badly are they cracking and perished would be my first question.
Perished and cracking sidewalls if too bad can severely affect the tyres rigidity and strength could be compromised. It could also be a MOT failure. (This I can remember from when I worked at the Michelin as a electro mechanical engineer and we had to do a 3-4 day course at ATS to learn about general tyre wear/safety)
My advise would be either nip into Krap Fit or call the mobile team stating you have received an advisory on all 4 tyres as the sidewalls are perished and started to crack. I dare bet if you called out the mobile team they would automatically change them at your home. If you ring mb and say you have received a advisory and you would like them changed for safety reasons they will likely give ou the go ahead.