Unfortunately, there is no a contract between Russia and Great Britain on legal assistance, recognition and performance of foreign judgement. However, the problem can surely be resolved. If your mother has already got the legal decision of the Russian court, after observance of authorization procedure of official documents she has the right to take legal action on recognition and performance of this decision via her lawyer in Great Britain. If in Russia the decision on the alimony has not been given, in my judgement, it is rational in this situation to charge the alimony straightway in accordance with the legislation of Great Britain passing the Russian juridical bodies.