When you say cover your bet, do you mean to protect your initial 70p stake or secure your £10.50 which you've taken from last nights result or indeed oppose the Hadjuk Split HT/Draw FT tonight so you will be in profit regardless of the outcome tonight?
Either way, you'll need to find out which price you can lay Hadjuk Split HT/Draw FT on Betfair and do the sums accordingly. For example if you can lay this price at odds of 19.0 (just an example price as I can't get on Betfair at work) you'd need be liable for £90 to win back a fiver!
So if Hadjuk Split HT/Draw FT came in you would have £157.50 from your Wm Hills winning less the £90 to Betfair Therefore £67.50 in Profit,
However; if Hadjuk Split HT/Draw FT doesn't come in you'll be £5 up less 5% Commission so that's £4.75 profit from Betfair, minus your 70p stake at Hills. i.e. £4.05 up
If I was you seeing it's only a 70p bet I'd let it run, rather than having to put a fair bit of coin into a Betfair account, also you would need to see what the actual lay price is for Hadjuk Split HT/Draw FT to work out the true values, remember though; the greater the price the more you'll have to be liable for!
Edited to add, I was typing this spiel before I seen fanny paddery's response!