Mint Cross Pollination and Spacing

I would like to plant several varieties of mint in my garden. I am aware they will need a barrier but will varieties cross pollinate. Is there a rule re the distance apart? I would like to plant some of the Westerfield mints this coming season along with the English and peppermint I have in two different areas of the garden but I would want them to stay true to form.

There are no easy answers to your question because fertility (the ability to produce viable seeds) among the mints is complex. Mints frequently can cross pollinate and produce seeds that develop into off type plants. Even when plants of only one mint variety are planted, it is possible that the plants will produce seeds of off types because some varieties are self fertile hybrids. There are known exceptions, such as peppermint, a naturally sterile hybrid incapable of producing seeds (so that beds of peppermint only will not generate off types).
How likely are off types in a bed of Westerfield mints, english mint, and peppermint? It is possible that off types will develop in this situation. However, the original plants will produce a dense mat of true type plants that will suppress to emergence of seedlings.
In practice, off types may not be a problem at all, or if they do emerge, it may not happen for several years until the mint patch weakens due to overcrowding and declining soil fertility. As a general rule we expect to replace mint beds every 4-5 years. (When replanting mint beds it is best to move the beds to a location where mints have not grown in order to ward off the build up of disease.)
Mints are bee pollinated so if you wanted to prevent all cross pollination you would have to separate the varieties by at least a kilometre.