A butane lighter works pretty good. A propane torch is also good, but be careful not to apply too much heat.
Heat shrink tubing is designed to shrink when exposed to heat - purpose is to provide a tight seal around electrical splices to keep out moisture and protect against contact with other surfaces. Heat shrink tubing comes in a variety of materials and this particular product is made of polyolefin which is rated to be good up to 275 deg F. Other heat shrink tubing materials are available which operate at temperature over 400 deg F. If your repairman used this specific product to provide extra insulation to the ignitor joint, I suspect it won't help very much. If he used a different brand specifically designed to operate at higher temperatures, it may provide some protection. In either case, it probably won't hurt and may help.