The intention is that you must accumulate a total of not less than 1 hour of training flying, ammassed in whatever you fly. So, if you hold both SLMG and SSEA ratings, 30 min in a SLMG, 30 min in a SSEA would be OK, for example. Or 60 min in a SSEA and nothing in a SLMG - or vice versa. Entirely your choice, it just has to add up to not less than 60 min in a 2 year period.
But if you are a Billy No-mates who only ever wants to fly single seat aircraft (this does not mean a 2-seater flown solo!), you want have to do any training flights.
The intention is also for 'consolidated' revalidation requirements for all 3 NPPL aircraft Class Ratings, so if you hold Microlight, SLMG and SSEA Ratings you will be able to revalidate all 3 by flying a total number of hours, subject to a minimum requirement in each class.
More on all this in a few days time when the formal proposals are released to the constituent organisations represented by the NPPL Policy and Steering Committee.