Here's what Steve Bennett said back in 2008, albeit not specific to the Hornby unit:
Minimum radius I would recommend is 6"/150mm, any tighter and you will certainly get problems with couplings, usually the governing factor rather than the loco's and wagons themselves.
Here's what the late Carl Arendt said even further back in time:
How do you make things work with such small radii? I’m just getting started and NMRA says minimum radius for HO is 14″ for 4-wheel locos (steam or diesel). I have an Alco RS3 I would like to try converting to On30 but don’t have the room for the NMRA minimum of 23″ radii (for the two 4-wheel trucks). Any help or direction you could give would be most appreciated.
Carl: Minimum radii are often in the eye of the modeller … don’t believe everything you read about them!
Most four-wheeled HO locos will easily go around 4 to 5 inch radius curves - or less. You need to work out coupling methods for the cars, but pins with links that drop over them will take VERY small radii. Alternatively, there are swinging Kadees that can be used. I have just finished a Gn15 layout that uses a loco based on the Bachmann On30 4-wheel trolley mech, with 4-wheel industrial-type cars, all of which easily go around 5 1/2"e; radii - using stock Kadee #5 couplers!
When you get to eight-wheelers (two bogies) you have to try them and see. The limiting factor on the curves is the amount of swing in the trucks (bogies), and some brands will turn much sharper than others. One suggestion would be the inexpensive Bachmann HO Brill trolley and San Francisco Cable car models — both are two-bogie mechs that turn on very
Basically you just have to get a length of flexible track and try out various curve radii. Sometimes trimming back the ends of the bogie castings can sharpen the usable radius. Sometimes grinding away obtrusive parts of the chassis helps, too! I haven’t done any experimenting with 8-wheeled mechs, but I have an old Athearn SW7 that will turn on an 8-inch radius right out of the box. And I bet with a little judicious pruning I could get it around a 6-inch radius.
IOW, the key is to experiment. Look at various locos in the store, take home the one whose trucks swivel farthest, and try out radii to see how small you can go. Then build a superstructure for that mech in your chosen scale and figure out what kinds of couplings will work. In general, shorter cars can navigate tighter curves, even with Kadees. Boulder Valley Models has some terrific On30 shortie car kits (both freight and passenger) that work great on tight radii (see http://hometown.aol.com/on30resinkits/
) or, of course, you can roll your own.
I hope that’s enough info to get you started. Good luck!