Sweet peas problem

Hi,

I have been growing sweet peas this year. I started them last year to go them off to a good start and then overwintered them. They were growing great (they survived storm Dorris where they got battered) i think because they has a good root system.

I planted them out in containers about 10 weeks ago against a wall at the front of my house (which faces South East) with trellis to grow up.

They took off great and really grew well. The containers are about 60cm wide, 15 cm deep and 15cm tall and i put in standard multi purpose compost, I planted 4 in one and 5 in the other.

They started flowering about 2-3 weeks ago, and they looked lovely, but the last 7-10 days they started to look a bit yellow, I thought it was due to lack of water (as its gotten hot the last week) so started to water more and using from a hose (before i used watering can from a water butt). The flowers on the one set (of 5) have stopped coming out and all the buds are yellow and dry to touch. The other one has been doing better but is starting to look worse as well.

I've read this could be over-watering and/or cold hose water instead of rain water so I am going to slow it down and swithc back to the water butt, but I've also read it could be due to too many in one container, either causing lack of nutrients or too rich? If i leave it a week and change back to the other watering method, and it doesn't improve do I need to add some nutrients, or will it be too late?

Thanks a lot in advance!