Trev,
Mike is trad and lives on a boat, therefore I suspect he may have problems getting set up with WFP.
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Anyway,
Mike, it may be better avoiding jobs like this if you're trad.
Real leaded windows are difficult to do to a reasonable standard using scrim or miro-fibre. They're hard work and time consuming.
Real-leaded windows differ from their modern counter-parts in that the lead used is much much thicker, therefore deeper, therefore the glass is set further back. It can be fragile to. I 'popped' a few small panes of the stuff out of a real-leaded hotel I clean while cleaning the inside (outside we use WFP).
Your options are:
1. Charge extremely well, and I mean well, so that you can take the time required to do a reasonable job.
2. Charge well, and 'manage your customer's expectations'.
3. Dump the job.
4. Underprice, do a top notch job; then dump it in six months when you've had a belly full of doing it.
I'd suggest option 2!