Any central heating installers on here? I need to add 1 rad downstairs. Is there any Rad to go for? In my mind they are all like for like but am i wrong? Are some better than others? It will be roughly a 600 x 1200 rad that i need. Whats the difference between a type 11,21 etc?

The options you have are
Single panel single convector type 11
Double panel single convector type 21
Double panel double convector type 22
The convector is the corrugated metal panel on the back of the rad which roughly doubles the heat output of the rad panel.
Obviously the depth of the rad increases the more panels/convectors you have
When I was fitting 15 years ago it was always myson round tops, purmo or stelrad
Myson do sizes in inches so if your replacing a old these can often fit
Purmo and stelrad are the grill type with stelrad soft lines being in my opinion the neatest
Mysons were always heavier so I took that to be thicker/better made

We used stelrad softlines when we did up our house. Nice smooth surface, very minimalist. Unfortunately they look like a massive whiteboard so our toddler decorated quite a few of them. Oh well....

My house, and a couple of others I've helped out on, have used Kudox rads. They're cheap, but i've no complaints from them currently, used with appropriate rust inhibitor. Screwfix had a deal on for a while and I ended up replacing all 8 type 21 rads in my house for a total of 86 quid, with a significant performance increase over the previous similar looking ones.

Thanks for the replies chaps

Excuse my ignorance....but....... the double panel rads......surely the rear panel is just heating the wall behind? Or is there more to it?

Ive no idea if it makes a difference, but the rad i plan to fit will be on an internal partition wall

Typical domestic radiators mostly heat by convection rather than heat (IR) radiation. You do get some wall heating but you can get reflective foil to stick on the wall behind the radiator to try and reflect it. The only issue with single panel radiators is they tend to have to be really quite large to be sufficient BTUs.
In my experience people often prefer radiant / infra-red heating and a colder air temperature whereas a room with hot air but little IR radiation tends to feel stuffy and yet cold. Almost makes you wonder why we still heat with convector style heaters! At work we're moving towards flat panel IR heaters.

PS typical advice is to put the radiator in the coldest bit of the room to cause optimum convection. That being said I think there's likely a balance between different types of efficiency. If you look at radiator efficiency people are generally talking about how much of the heat in the water gets radiated into the room. To me that's less of a concern (just fit a bigger rad), the concern is more how much of the £s spent on heating the water leaks out the room.