Replying to [fazUK]:
It is all down to your business plan, if you want to do long term business with a supplier in China, so the trip to China is great start, to meet the supplier and talk more about your business and this would improve trust on both sides.
To have a Chinese person with you is okay, but you can understand the supplier from the first things you see at the site, if you was going to be there at a direct date, you can ask for someone to collect you from your hotel and have a person there to speak English, so again this is where a Chinese helper is good.
To buy samples is one thing and to buy more and ship them, as you know you need to have CE and nice with Rohs too for UK.
In Shenzhen you find many suppliers for security items, we have just bought IP camera, wireless IP cameras, baby cams, walkie talkies, and we like to test them for a while to see how they work, and the manual, carton, packing, software and so on.
Whne you are in Shenzhen you can travel to H/K via train, make sure you have a mutiple visa to get back to the mainland, or fly to H/K first then Shenzhen and back to H/K.
The canton fair is a good place to go, you would see many things there, we got to see many suppliers for security goods, and its only 1.5 hours to Shenzhen from Guangzhou.
Find the right supplier is paramount and then you can think of long term business. good luck