Thanks Fred and Barry
Thank you both for well wishes. Hope you and yours are all well.
Fred! dare I mention the supplier (Brownhills) Free postage and 10% discount. I haven’t received it yet so things could go pear shaped.
I don’t like the idea of squeezing a runny product into the gap nor do I want it to stick fast in case I want to remove it. It doesn’t have to be super-watertight as it is inside and there is a rubber seal on the top of the frame. The nearest thing I found is W4 Mastic Sealing Sealant Strip https://www.amazon.co.uk/W4-00017-Mastic-Sealing-Strip/dp/B0036KG89I. Oliver Dill, whose roof light has only 2 lights so different procedure, did not replace the mastic so it was probably a tighter fit.
Also there appears to be a thin sticky tape above the frame probably for the seal. I will probably use silcone grease here.
Tom&Carol