What you're experiencing is normal to TURBO cars. This isn't even an issue of Lotus. Popping is due to excess fuel in the exhaust, exploding. I have a 3inch catless setup on my MR2 for example. It pops quite abit on decel and sometimes at idle.
As to why just the passenger side, depends on the way the exhaust is plummed. If that side has a shorter distance or run then it may just be a natural occurance. For my own car, my right tailpipe exhibits more popping/flames due to it being the natural route exhaust flow takes. Especially if you have unequal length manifold design.
I'd say your car is fine and if it annoys you then look into leaning it out more, causing less excess fuel in the exhaust. Don't know for the Esprit as to what your aftermarket ECU options are but it's probably the only way. Many stock turbo cars run rich for saftey. Enjoy!