I would agree with avoiding cheap eBay manifolds
There are a few tried and tested solutions.
BBR catted £822 or non-catted £582 all plus shipping
IL Motorsport £419 plus shipping, Thrussington Garage and Skuzzel recommended.
Racing Beats £694, Roddisons recommended
I was just about to buy a second hand Racing Beats when I noticed MX5 Parts has a 10% off sale so I have a IL Motorsports one sitting here waiting to be fitted cost £377
Autotronix in Sheffield will be fitting and remapping the car in two weeks