D.I.Y Roof repair?

I have just had my loft boarded out. There was some heavy rain and I found some water dripping off some underfelt that has come loose next to my chimney breast. It may have been leaking a while but I had a lot of insulation in place before. Would it be enough for me to tuck it back around the chimney and perhaps but some expanding foam to plug any gaps? There doesn't appear to be any slates missing from outside. As a single Mum who is a willing D.I.Y. enthusiast I would like to try and do it myself or am I just being a little naive? I would be grateful for some guidance please. Thank you.