Hi there I work as an employment lawyer and would be happy to give you advice on this matter. I will address the questions as best as I can but for clarity it is impossible to give a definitive answer on this matter, but I will explain why..
Question 1 : My questions is what do I do when I receive the threatening letter from the solicitor/current employer? The standard practice would be to take out an injunction to stop you from breaking the restrictive covenant however, if you accept the job the may look to take legal action, at this point you would be best advised to instruct a local solicitor who specialises in this area.
Question 2: Now that I am on gardening leave , can I join the new company after my gardening leave or notice period is over? Once you garden leave/notice expires then you are no longer employed by the first company, DO NOT accept the job whilst on garden leave or you would be in breach of contract and that would cause additional issues.
Question 3: I will be selling their new products in the UK which do not compete with the current employer's product and other products in India and other regions where I have not worked or sold before? Will this strategy be enough to get away from the restrictive covenant? I will address this in my response below.
Question 4: What should the new employer do to ensure that they are on the safe track to hire? Is there any precaution you would recommend? The restrictive covenant binds you and not the new company however, you may wish to make them aware of the situation if you are confident it will not affect your employment.
The fact about restrictive covenants is that they are not always enforceable even if the first employer takes legal action against you. From the information you have given it would appear that your new employment could not reasonably have an effect on your previous employers business on the assumption you will not be breaching any contractual obligation in relation to other aspects such as "inside knowledge". Previously companies have tried to enforce restrictive covenants and failed due to the covenant being too "wide" in their application. Also given that you are working in such a specialist area, there is case law which states that if you were to be unable to work as a result of the RC then it cannot be enforced. Given the limited work available to you in your area then it would of course be advisable to take the job. However, your previous employer may with to take legal action against you. If they do this you would have to instruct a solicitor to deal with this matter (employment lawyer) you would have to go to a tribunal and give evidence to show reasons as to why the covenant cannot be enforceable on the grounds of being too wide in its application and that you would suffer a detriment as a result of the enforcement of the RC. The reason I state that I cannot give a definitive answer is due to the fact that a final decision would go on the merits of your presented evidence. I would be irresponsible not to advise you that there is of course risks in litigation but there has been quite a few cases where the court have decided that the RC's cannot be enforced.
I hope this has answered your question. I would be obliged if you could "accept" the answer on the webpage if leave some feedback should you have the time. I wish you all the best with your situation.