i,ve done my electrical training as part of my engineering qualifications - if he,s replaced a 15A fuse with a 30A one this will not "Cure" the problem it mearly allows more current thru into your house - not sdeal or good practice but still it wont cause any overheating too badly of your wall cables as they are rated at lot higher. you say you,ve unplugged everything in grow room? try and do it like this go round all the rooms in the floor your grow rooms on count all the appliances and equipment you have plugged into sockets or outlets - it may be a case of you have too many thing plugged in around your house
i.e a ring main circuit (your sockets) is rated at 13A so you can plug things safely that are up to 13A however you breaker will be rated higher than 15-20A - bear in mind a tv -dvd player -clothes iron and hoover and washer dryer , fridge and freezer all run at same time in most ppl,s house and thats under the 15A of the breaker you may just be pushing it near the limit
hope it helps -althiugh a bit long winded