Mel said:
Never mind all this talk of duct tape.
What I need to know is what is the best way to get the old mastic off, and what mastic do I reseal with.
Thanks
Mel
Try this MEl, just go easy so as not to gouge the substrate, https://www.toolstation.com/window-scraper/p53477?r=googleshopping&rr=marin&utm_source=googleshopping&utm_medium=feed&utm_campaign=googleshoppingfeed&gclid=EAIaIQobChMI2LXKqcHh3wIVwud3Ch1_aAuxEAQYAiABEgKUb_D_BwE
Once you have removed all the residue of the old sealant then white spirit and a sponge scourer (the one you probably use for doing the dishes) should finish it nicely.
I don't want to contradict otherclive, but white spirit is a powerful solvent and should do a really good surface preparation job, if the cleaned surface is smooth and looks clean then I don't think meths is necessary.
You also don't need a 'joint' sealer', so this should do nicely to fix the new flap cover:
https://www.toolstation.com/premium-universal-acrylic-sealant/p66598?r=googleshopping&rr=marin&utm_source=googleshopping&utm_medium=feed&utm_campaign=googleshoppingfeed&gclid=EAIaIQobChMI7tSl4sHh3wIVC7DtCh0k4QgWEAQYAiABEgJZP_D_BwE