Charlie, I did Manchester in 2013 and really enjoyed it. It is the only marathon so far where my pace didn't suffer drastically in the last few miles. I think 15 secs slower in the last mile or so was it. Probably due to flat nature of course. Organisation, medal, goodie bag great and you run from theatre of dreams so parking not an issue, car parks are plentiful and can be booked in advance via man Utd. I really enjoyed it! They had improved since the previous/first year which was not a good restart to th Manchester marathon and fantastic reports ever since.
OMG Eggy, 20 miler 3 days before a half PB! I was worrying about 20 in the week before Silverstone but figured that'd give a longer taper to get the miles out of my legs for Paris. A shorter fast half is less taxing on the legs than a marathon pace 20 miles (again I'd considered doing the 20 at MP+30s to compensate). They do say taper should be less mileage with increased intensity ... I would have thought less likely to get injury at Half marathon pace compared to 3k track pace albeit a much shorter distance (and hence the need to warm up properly)
I guess there's no right or wrong answer. Quite a few people do Silverstone in the lead up to London (which is 3 weeks later than Paris this year).
I'm giving it a try this year and as Paris will be my 10th marathon I'll be able to see if it impacted negatively or positively.