>>>When a witness/victim makes a statement what are the reasons for them to make more than one recounting their version of events?
You'd have to ask each witness/victim for their own reasons (which could be different to those of other witnesses/victims) in order to answer that!
However it's not unusual for people to want to add to, or amend, statements which were given 'in the heat of the moment'. For example "I said that it was the guy in the red jacket that kicked me when I was on the floor but, now that I think about it, I only know that it was him that pushed me to the floor. It could have been one of the others who actually kicked me".
>>>When making a statement is there a declaration stating that the information is the truth to the best of their knowledge?
That's implied anyway. Anone making a false statement to the police risks being charged with an offence of wasting police time or (far more serious) of attempting to pervert the course of justice.
>>>Why would police not photograph alleged injuries?
Simple practical considerations might come into play here. e.g. they'd not got an officer with a camera immediately available to take such photographs and/or they'd got no way of knowing whether the injuries exhibited by the (alleged) victim were sustained as he/she suggested or possibly before or after the alleged assault.
>>>Are police allowed to withhold statements which conflict previous statements made?
The police (per se) aren't specifically obliged to provide information to anyone. However the Crown Prosecution Service must ensure that all available evidence is handed to a defendant (or to his lawyers).
>>>Can police pick and choose what witnesses they are taking statements from or do they investigate one side and disregard anything which potentially throw doubt on witness statements?
Unlike, say, France (where the law requires that an 'investigational' approach is adopted) the English judicial system is 'adverserial' (whereby the two sides are required to put forward different arguments, for and against conviction). Both the police and the CPS are meant to examine ALL of the evidence available but the nature of the system means that they both tend to concentrate only on the evidence which might support a conviction.
>>>Can police pick and choose what witnesses they are taking statements from or do they investigate one side and disregard anything which potentially throw doubt on witness statements?
See above!
>>>Is it necessary for an alleged victim to provide medical evidence/records? If not then can the defence insist on this being provided?
There is no 'rule' that medical evidence must be provided but, where there is doubt, a defendant's legal team would be right to insist upon it. (e.g. "M'lud, we have heard that the alleged victim suffered really serious injuries but where is the evidence to support such a claim? I submit that my client cannot be convicted of causing grievous bodily harm unless there is PROOF of such injuries").
>>>Can a defendant produce evidence at a trial or does the prosecution have to introduce it first for it to be used?
A defendant can use any relevant evidence to support his/her case. However the 'alibi rule' means that certain evidence must be presented to the CPS at least 14 days before the trial, so that the prosecution has a chance to examine it; otherwise the court can refuse to accept such evidence. (e.g. a defendant can't simply say, with no prior warning, "I was at my mate's house when the offence took place, so I can't have done it. Here's my mate to prove it". The police and CPS need time to check out such an alibi).
>>>Why would police refuse to investigate counter allegations?
Probably because they don't believe them! (See also my reference to to the nature of the English adverserial system, above).