Before you can make a citizen's arrest, you have to be sure that the maximum penalty for the offence is, or exceeds, 5 years imprisonment. (Theoretically that also applies to the police but the Police and Criminal Evidence Act, together with other legislation, gives the police additional powers which don't apply to the general public). See here:
http://www.bbc.co.uk/crime/law/powersofarrest. shtml
There is no statutory definition of reasonable force but you might find this of interest:
http://www.lawinabox.net/lbnewswire07f.html
It should be noted that some police forces have concerns over the use of 'reasonable force', to such an extent that they have banned Community Support Officers from using it:
http://news.bbc.co.uk/1/hi/england/lancashire/ 4365611.stm
Chris