Yup, patch on the inside. Doesn’t need to be anything special, a normal large patch out of a puncture repair kit will work providing you do a few things. First, roughen the area with sand paper and make sure any ribbing or other moulded in texture is removed, then clean it very thoroughly with meths. Let it dry then smear a thin layer of vulcanising solution on the inside of the tyre, let it get dry to the touch, add another thin layer and this time just as the vulcanising solution has got so dry to the touch that you’d think it’ll never work, peel the backing off and stick the patch on. Rub it down very, very thoroughly and leave it to dry for an hour or so. Job done.