Double Glazing into Sash Box

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Hi

I've fitted most of my windows my self but need a bit of advice for my last one.

I'm replacing the old sash window in my back bedroom and wanted to fit a UPVC double glazed unit. Ideally I would have replaced with a UPVC sash but at well over double the price I really can't afford it so this will have to do.
Although the windows are rotten the sash box is still in a very good condition.

I'm thinking of having the window made to fit into the box which will save me loads of time a hassle plus not having to re decorate the bedroom.

Is this ok to do ?
Obviously I will clean up and paint the box first once the windows are out (just have to remove the staff bead?) but wanted to know if there is any reason not to do this or if anyone has any tips.
Getting window made by http://www.tradewindowwarehouse.co.uk, anyone had any dealings with them ?

Any advice would be great
Thanks

 

[God]

we glazed a entire pub this way, so much easier and cleaner. Only downside i can see is if the box did rot you will have to take the window back out, but really we have had no trouble.