Helen - If you feel your leg is 100% healthy, I would go for it. From what I have read from other London Marathoners, the weather was also a bit of a factor? Looking at your HM
PR, I really think 3:45 is well within your reach. Trust me I know the allure of "good for age" (or as we call it in the US "Boston Qualifying"

) It really feels like a whole other level ...even though really at 3:48 you are a goddess
If I manage to take off a little more time on my HM I'll be pushing for BQ in the fall and I think if I came in at 3:48 in PDX and was healthy, I would do Tucson 8 weeks later to try again on an easier course.
Of course I am sitting out this week trying to nurse along a few niggling injuries because I finally stopped being stubborn about them healing on their own...and what happens, I get a NEW one. Suddenly my right shin is shooting random pains. Buggers.