You
might be able to apply, under Section 50 of the Administration of Justice Act 1985, for the dissenting executor to be removed but you'd need to show that his obstinacy was prejudicial to the interests of the beneficiaries. Such an application is unlikely to be something that you could do without the help of a solicitor but it might still be worth considering:
http://www.step.org/tackling-troublesome-executors
On a more practical level, why not consider if there are any ways of marketing the house that don't involve the need for estate agents? I sold my late father's house simply by putting large notices in the window and advertising on supermarket notice boards, without the need for an estate agent. Perhaps a similar 'DIY' approach might be useful to you?