Sash Window Locks - Restrictors

[Johnsmith]

Hello. Want to fit some locks onto my original wooden sash windows throughout the house. Have been told the easiest and best way is to fit two restrictors either side of the bottom sash (to stop them being opened or closed). Is this the case? Had a quick look at my local locksmiths and have been told they'll cost £10 for a set of 2 per window. Seems a bit steep for a tiny bit equipment, especially as I have nearly 20 to fit! Any suggestions for other ways or where I can get them for cheaper? much appreciated. Cheers

 

[ouch!]

http://www.screwfix.com/app/sfd/cat/pro.jsp?id=11260&ts=77159

I'm toying with the idea of these at the moment.
They look dead easy and are a fair bit cheaper.

 

[Johnsmith]

a friendly locksmith has told me not to bother with those ones but to use external ones instead. Reason being that, as the windows age, the holes will likely fall out of sink, making the locks impossible/difficult to use.