Anyway, once the journey was over, we actually did all successfully meet up - couple of phone calls required to locate each other, but we did in fact recognise John (having last met him about 10 years ago or so - before we moved house, anyway, so more than 8 years) and he recognised us, so that all worked out ;)
I wasn't sure if they'd have a plan, coz I'd been singularly unorganised (somehow the summer had seemed overwhelming back in June, so I was organising things on a one at a time "after this then that" basis, which meant this trip was only thought about after Nat's birthday). But in the morning I'd had a quick look at the map and planned out a brief walk round iconic London sights, just in case there was no other plans, and crossed my fingers for good weather coz most indoor type tourist things seemed to shut by 5, so too early for us.
We ended up following my plan - and it even didn't rain much (we did shelter under a bridge for a bit to let the heaviest shower pass over tho). So with the help of my trusty A-Z I took us from Leicester Square down to Trafalgar Square, then via the Admiralty Arch up The Mall (with a detour through the edge of St James Park to get ice-cream & see some greenery) to Buckingham Palace. Then along to the river, past the Houses of Parliament & Big Ben, and along the Embankment looking across at various buildings including St Pauls (& the London Eye) to Cleopatra's Needle (that was for J's benefit primarily). And back to Covent Garden. Where everyone declared themselves walked out, so we didn't go look round the shops etc there. John & Cressa seemed to appreciate the little tour, so I felt quite pleased with myself ;)
Food was a little less well organised - we so rarely eat in London when we're not going somewhere, so we don't have favourite restaurants or anything like that. And I'd ended us up right near Covent Garden in the middle of theatre land, so places were pretty busy. Tried to get into Cafe Rouge where the waiter very politely didn't laugh when I asked for a table for 6, but I could tell he wanted to ;) I'd spotted somewhere that did fancy sounding burgers tho, that didn't look full, as we walked past. So we headed back there - called Byron, and in retrospect it was a bit too US themed to take North American visitors to (even if they were Canadian), and whilst the food was filling and tasty, I wasn't inspired by it so will likely not go back.
But still, while sitting there I had a sudden inspiration about what to do with the rest of the evening - it was a given that it'd be spent in a pub, preferably with proper beer (coz John is also into his beer - part of their holiday so far had involved going to Swansea for a beer festival). And I suddenly realised that we weren't that far from High Holborn and thus we weren't that far from the Princess Louise! So after we were done with our food we headed up there, and lucked out & got seats too :) The rest of the evening was therefore spent sampling the beer there (well, those of us able to have a beer - J & A were both driving so stuck to the soft drinks) - and I also bought Cressa a pint of shandy coz she'd wanted to try it but John wasn't ordering it for her coz it was a bit of a girly drink ;)
And then home - a good day :) Nice to see John again & nice to meet Cressa. (Nice to see A & P too, but we see them more often ;) ).