DT Your spread bets are laid off in the market. Your two £300 a point purchases went through the market this morning. So Spreadex are holding physical shares on your behalf. At a rollover they do not sell them and buy them back. They simply make a book entry in their ledger and charge you just over a penny a share for an extra three months' time value. So with a six months-to-expiry contract, you pay upfront double the time value you would pay upfront for a three-month contract. There is a slight cashflow advantage to using the three month contract, but it's pretty marginal.
Don't be scared by market tightness when you roll-over. You can pick the timings on your roll-overs. You could roll March now into June if you were so inclined. But once the spread firm holds the actual stock, to cover your position with them, you can roll-over until the cows come home so long as you keep your margin intact. NB It's worth negotiating with your spread firm the terms of any roll-over. Do not pay more than a tiny fraction (say 0.1p) over and above the fair time value (say 1.2p).