Passive Stations were installed as part of the Global Positioning System (GPS) 
The OS Passive GPS station database:
Ordnance Survey's network of passive GPS stations throughout Great Britain allows GPS surveyors to precisely position their survey stations in the ETRS89 coordinate system . OS passive GPS stations are geodetic quality ground marks in user-accessible locations. A typical survey site in Great Britain will have several OS passive stations within 20-35 km (distances may be greater in the Scottish Highlands). The main differences between this network and the Active station network are that passive stations must be occupied by the user's own GPS reference receiver during the survey; and that typical distances from a survey site to the nearest passive stations are smaller than for Active stations, making required observation times shorter and allowing the use of single-frequency GPS receivers.
The OS Passive GPS station database (List  or Search ) is a searchable database of all passive stations, including precise geodetic coordinates, station descriptions and photographs, location sketches, access information, and recent maintenance and monitoring details. The database allows GPS surveyors to plan control surveys based on reliable OS control stations. To use the National GPS Network passive stations, two or more survey GPS receivers (preferably dual frequency) and processing software are required. The resulting ETRS89 GPS coordinates can be precisely converted to National Grid coordinates and Newlyn height coordinates using OS precise transformations.
Physically, Passive Stations take a number of different forms:
- Berntsen (104) 
- Bolt (88), often but not always located in a concrete Block
- Buried Block (at least 2) ie Gallows Hill http://trigpointing.uk/trig/57 and Bolnhurst http://trigpointing.uk/trig/755).
- Cut (2) ie Hugh Town http://trigpointing.uk/trig/7712 and Down Barn Farm http://trigpointing.uk/trig/311
- Fundamental Benchmark (201)
- Pillar (231)
- Rivet (143)
- Surface Block (237)
- Total 1008
All of the Fundamental Bench Marks are designated Passive Stns, with the exception of 6 that are recorded as Destroyed. And all of the 104 Berntsens listed on T:UK are Passives. FBMs usually have another Passive in fairly close proximity, typically consisting of a Berntsen and/or a Block.
An example of an FBM with Surface Block and Berntsen close together is the Oswestry FBM http://trigpointing.uk/trig/7182 with nearby Oswestry Aux 1 Berntsen H2SJ2724 http://trigpointing.uk/trig/606 and Llynclys surface block C1SJ2824 http://trigpointing.uk/trig/181 ; they are 0.4km from each other.