SOS my Senetti is struggling

I bought this gorgeous purple Senetti from Homebase about a month ago. It didn't come with many instructions. Firstly I potted it and put it in my front garden where it quickly became battered by the wind. So I moved it to a sheltered area and all seemed to be well. But now a month on the flowers seem to be struggling. I keep it well watered and have been regularly feeding with a general npk. No signs of slug attack although I did notice a few ants on the flowers and some of the petals have dark spots on them.
I've been dead heading the unhealthy looking flowers but if I carry on its going to look like Morticia Addams' plants on the Addams family
Will post some pics.
Appreciate any advice you have guys.
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Here you can see some of the flowers are wilting. Some are also paler in colour.
Some of the petals wither away. They don't appear to have any bite marks however.
This pic shows the patches appearing on some of the petals.
You might have been over-watering, especially as the weather is still not very warm. While the temperatures are low let the compost dry out between waterings - lots of useful info on this site http://msucares.com/news/print/sgnews/sg11/sg110215.html
Good luck
Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.
Thank you Dovefromabove, that would make sense as I may have been a bit over zealous with the watering can. Thanks for the link, will check it out now.
Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.
Thanks @verdun, if the situation doesn't improve then that's what I'll do.
I have one of these in a pot and I'm dying to cut it right back as it is looking very sorry for itself. The problem is that I have a nest of tree bumblebees and they still absolutely love it! It looked fabulous for a good 12 weeks after I bought it, but is now really struggling, however I wouldn't hesitate to buy another especially as it has been so popular with bees and hoverflies.