I have this at home, and its well worth a read, i use it all the time, its part and parcel of owning a british car...rust
http://www.haynes.co.uk/webapp/wcs/stores/servlet/ProductDisplay?catalogId=10001&storeId=10001&productId=30053&langId=-1
did you remove all the rust on the arch before filling it in, if there is any rust even if you just put some krust on top will still spread up from under all that expoxy. its a good effort, if your going to tackle tasks like that, its worth investing in a mig welder and teaching yourself to weld. and also an angle grinder.
your top patch on the arch doesn;t look bad, although it protrudes slightly, but the pop rivets are a little worrying, are they counter sunk below the body? you might get away with it. but you might get sagging ontop of the rivets which will make them visible later.
heres my wheel arch, this was doen with a spot welder,
ok this is the other side, but it had almost identical treatment, and this is post blending and painting
on this occasion this is not my welding, but i have cracked out the kit for other parts on other cars.
but that haynes manual on bodywork is very good
then ground back ad then finally filled in and blended.