“The overriding objective [of the CPR] is that the court should deal with cases justly. That includes, so far as practicable, ensuring that each case is dealt with not only expeditiously but also fairly. Amendments in general ought to be allowed so that the real dispute between the parties can be adjudicated upon provided that any prejudice to the other party or parties caused by the amendments can be compensated for in costs, and the public interest in the efficient administration of justice is not significantly harmed.”