There are several threads on the Lew W.-style hydraulic brake mods, egviewtopic.php?f=4&t=4184&p=26011#p26011
I'm intrigued on the earlier reference to using Metro brake shoes, being the owner of a pair of Metro drum brakes myself. The basis of the hydraulic 2LS set-up is a Mini 2LS front brake plate (near-side probably!), wheel cylinders and brake shoes (but you can see in the above thread that some have an alloy mounting plate for the wheel cylinders).
Mine has ended up with a lot of extra weight inside the Ariel alloy plate and the steering is noticeably heavier (I wonder if David has noticed that on his conversion).
The BMW wheel is for /5 or /6 bikes, I think the brake is the same for both, the hub might have different numbers of ribs around them (very important to get the right ribs to match the year of the BM!), more importantly there are 2 different spindle sizes, 14mm and 17mm. The later ones would be 17mm and that was my choice when I bought one of these 'cos it's the same as my R100RT and the front wheel bearings have done pretty well on that (hope I haven't jinxed it as reputedly they're a b*gg*r to replace).
There's a selection here:http://www.motorworks.co.uk/vlive/Shop/ ... 0&spPage=1
Not cheap but it's a 200mm 2LS brake and Dick W. reckons it works pretty well (and mine seems to have arrived with an alloy rim as well)
I've not fitted mine - it'll need the later sliders from the 1" axle Ariel hub with some cylindrical spacers around the 17mm axle and probably a top-hat thingy somewhere to centralise it. (I did ask Disk if he had any dimensions for the spacers but I got the impression he freehanded it...)
I reckon I can use the BM brake steady but I think Dick's used the Ariel one (or made his own).
I've never had a grabby FWH brake, front or back (although I've only ever had the one of each). The hydraulic 2LS is a definite improvement.
'55 Huntmaster, '56 VH, ' 51 VH, '80 R100RT, '00 Sprint ST (but all those Ariel parts can only make one running bike...)