Shorting the housing market and buying coal

Many of the housebuilders shares are falling in price and crossing the weekly 50ma line and triggering sell signals. However, I'm having a problem opening long-term sell positions in these companies with my spread-betting platform. They refuse to open positions because of problems with not owning the underlying shares. So what I have done is short one of their indices - the 'Household Goods and Home Construction' section of the FTSE 350. Because there are no underlying shares involved, it seems to be easy to open a short position in this derivative.

I have opened a long position in Peabody Energy - a US coal miner. They supply metallurical customers - another indicator of the rise in industrial metals, perhaps?