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Cost of entry is the thing putting most people off. It's difficult to justify the cost of the specialised skeleton gun and the tubes are £50+, primer is expensive too. Totally worth it if you are doing repairs (especially exterior) professionally.
County Chemicals have a cheaper option for smaller jobs :-
https://www.sealantsonline.co.uk/Products/3C-sealants-wood-repair-system/CCC0083
That kind of repair is a good candidate for epoxy resin.