Mark frv

Hi. How easy is it to replace the exhaust on an Frv. Most of it is stainless steel but the part the mechanic welded is not. I think it is just the one downpipe that needs replaced and am wondering how hard it is to do and where would I get the relevant part. Apparently the mechanic said there are only OEM parts available and it will cost about £400. Sorry I'm a bit vague but that's all I have at the moment