Personally, I am tempted to go with this kind of set up:
http://www.flickr.com/photos/7612786...2842/lightbox/

She locked the chain with a padlock which is the blue thing under the seat. I was thinking of getting something like the £10 Abus chain (i think it's 8mm links and 90cm in length) with an integrated lock. As least it isn't going to be as easily cut as something I would use with the stock seat on my QR seatpost like the kryptonite saddle saver cable.

The only downside I can see is trying to find a chain which is exactly the right length to hook from the frame round the rails of the Brooks. Then it's a matter of using some tape to make sure it doesn't shift around and damage the frame, and maybe like her, cover the padlock with some tape too and somehow affix it under the seat so it doesn't interfere with how the saddle functions. Also the plastic bag when parked is a must in UK anyway.

I get my replacement bike next week after saving up for 8 months after the last was stolen (insurance didn't cover the whole cost). The saddle post on it is quick release and rather than spending £8 for a cable which might be replaced with a chain when I get the brooks, I'd rather sort everything out now...i.e. custom bolt vs chain like in the pic? Where exactly can I get a custom security bolt in the UK which will be suitable for both the stock saddle on a Marin Muirwoods 29er and the Brooks B67.

Is this good enough:
http://www.freemanscycles.co.uk/prod...20Seat%20Bolts

Or even this set?
http://www.evanscycles.com/products/...t-set-ec011572

Not sure if the bolt is for the seatpost or the saddle itself. I have QR wheels/hubs, so I don't mind getting a kit since it would prob be good to replace my QR with security skewers/bolts at some point.

By the way, does the below imply that these kind of bolts are not so secure?
http://answers.evanscycles.com/answe.../questions.htm
Anyone know if these security bolts would be easily removed into using a 6" nail and a pair of molegrips?