You are certainly not alone; up-graded loft insulation has caused excessive condensation on the undersides of roofs nationwide.
The highly commendable improvements you have carried out, could of course also have contributed to the problem by further containing and channelling warm moist towards the roof. Nevertheless, these improvements are obviously of some importance in enhancing your living environment and so the problem has to be dealt with, and the only answer is improved ventilation.
It seems your best solution would be lapVent in respect of costs and execution, but how many and their positioning would depend on a clearer picture of the situation.
They come in pairs including a complete fitting kit; you just have to supply the tools. At least two are needed in order to provide an all important inlet and outlet arrangement. In addition, lapVent is extremely adaptable, perfect for your purpose as just one pair could be trialled and add to or not as seen fit. In your case and if the roof space is only small, 2 mounted centrally and spread as wide apart as possible could do the job.
Contact Tel: 07894 459288 for further information on your other queries.