Ok, removing the battern that is supposed to protect the window from slipping has revealed rot to an extent that just shouldn't happen. I'll Ronseal wet wood rot, paint over it with weathwrshield and replace battern a bit lower that the factory level.
Jesus wept, you have numerous tradesmen offer you advice for over a year. You ignore their advice... and on the probability of balance you will be back here again in the winter, complaining about the rot.
You either need to replace the timber or splice in new timber.
Sorry, I don't like being combative on this site but... quite frankly you are wasting our time.
Accept advice or assume that everyone trying to help you is lying. The choice is yours, but I really can't be azzed to engage in this circular converstaion any further.
That said, I genuinely hope that you accept appropriate advice. Best of luck.
edit--- my post was indeed combative, for which I do apologise. I stand by the point I was trying to make though.
If you want advice with regards to timber splicing or replacing timber, just ask.
If you disagree with the advice offered here, that is fine. Each of us might offer slightly differing solutions but if, absent of a valid reason, you ignore all of the advice, you may well find that none of us professionals want to give up our free time to help you. I am sitting in the pub typing this after a long day.
I am a decorator by trade- I often have to splice in timber with epoxy resins. At the time of re-posting this,
@crank39 (was the last person to reply). He is a glazer whom I often defer to when I need advice about windows and doors.
This site is free to use but it would (probably) not exist if us tradesmen were unwilling to offer free advice. When people seem to ignore advice from professionals it makes us (read: me) less inclined to help.
I genuinely hope that you take the above on board.
Best of luck