Planned a trip from Fenland to Sandown IOW everything featured in the PLOG except a radio frequency for Solent/Southampton. Southampton VOR/DME details recorded fine. Has anyone else experienced this or indeed any other missing frequencies?

Can you say why you were expecting the frequency to be shown? Were you penetrating the Solent CTA as part of your plan?

I just tried creating a route that flew all the way around the Solent/Bournemouth CTA about 1nm outside at 3000', and while various Southampton and Bournemouth ATS were listed, Solent Radar was conspicuous by its absence. As soon as a leg crossed into the CTA though, Solent appeared in the PLOG frequency list. Curiously, a very similar flight just outside the Gatwick CTA at 2000' does include Gatwick Director on the PLOG. I would certainly expect any flights passing within (say) 5nm of a CTA to include the frequency of the CTA just in case you become uncertain of your position...