"If those 243,000 net immigrants have been trained abroad and now working paying tax in this country and plan to go back to their own country in retirement then why is it a problem?
No-one has yet come up with an alternative solution to the pension deficit other than immigration of young working people."
Look, while some immigrants may live in the UK for a bit and then go home (in which case I would classify them as guest workers, not true immigrants at all), others will not; they will stay, and then they will then age in turn and require even more immigrants to sustain them, and so on and so on. Unless the UK can take an ever-expanding population this really is not viable in the long run. It is a temporary windfall that simply stores up the problems for later.
The only long run solution is: a) improvements in productivity which enable more wealth to be produced even when the workforce is not so large, and b) changes to the pension system. Both these things demand some effort on behalf of society, and from a political point of view it is understandable that politicians prefer to choose the easier option of importing more workers. But it won't work for ever.
The "bring in more immigrants to solve the pensions crisis" makes a reasonable amount of sense in countries where the fertility rate is chronically low, like Singapore or South Korea; it doesn't make a lot of sense in countries like the UK, where the fertility rate is actually fairly OK and the real issue is the design flaws in the pension system.
See here, for example: cep.lse.ac.uk/conference_papers/28_11_2007/TunerImmigrants_Slides.pdf (Adair Turner, LSE)