Which dinghy best matches my requirements

I am a 46 year old relative novice and I am looking for a boat which does not exist unless you advise otherwise.

I don't think a grad or a N12 would be a good idea for you at your level of experience. They're both out-and-out racing dinghies and the N12 is rather tippy. You might not mind ending up in the water but what about your children? Could you cope with a panicking child and a capsized boat? Fevas and Magnos are just pieces of plastic crap IMO. So really you're down to the Miracle and Pacer, both designs by the late, great Jack Holt. Probably not much between them. I'd also recommend that you take a serious look at the Heron. Although it's smaller and slower than the Miracle or Pacer, it's a lot more stable. It was also designed to accommodate two adults and two children so should fulfil your capacity requirement.

I would say be realistic about the number of times you will sail the boat more than two or three up.  If you really want to sail with four or more then (and I know its a bit heavier) thinks about a wayfarer.  They can still race well on handicap even though they might fall in your big old bucket category.

You probably won't that many people in an RS Feva to be honest. You could have a look at the RS Vareo? I'm not sure about how much they are but they meet your requirements. A Laser Vago or Topaz Magno/Topaz Xenon would also work well :)

I highly recommend the Miracle. It can be used as a family boat which can take a combination of people including two adults and two kids at a squeeze.  It can be sailed single handed with or without the jib easily or with two persons for optimum performance.

Its light to launch single handed, fast enough to be fun, easy and safe.  There is a strong class association and you'll probably find them at most clubs around the uk.  If you're not after a wooden boat, which in my opinion can be fun to maintain if you've got the time, there is a GRP version which would be a bit costlier.  there is also a compostite GRP hull with wooden deck version. Butler Boats are the manufacturers.

Have a look at the Miracle Class Association website for more information and maybe get to try one out for yourself.   I learnt to sail in a wooden Miracle and despite moving on to a faster boat I still have a lot of respect for it.

Regarding the other boats you mention.  My opinion is that the Feva would be too small, the National 12 too challenging, The Magno pretty good for size and low maintenance with the option to use a kite but maybe a bit heavy to handle on land, Graduate would be heavy, Pacer - don't know anything about it I'm afraid.

Hope to be of help.