If I am the petitioner, on the grounds of unreasonable behaviour; do I get landed with all the legal bills?

I'm worry these could run into the 000's and he will get away scott free, again.

You must make a distinction between, on the one hand, the costs of the petition itself - which are relatively modest ( mainly fees ) and you might get exemption from fees if you have a low income ( which, since you mention legal aid, I assume might be the case ).

The cost of proceedings for ancillary relief are another matter entirely. They can easily run into five figures.
The general rule is that each party pays his or her own costs.

Even though you may be on legal aid, this is a loan, not a gift and may have to be repair out of the settlement.

For these reasons we usually recommend mediation if the issues are straightforward the parties are still on speaking terms. It is perfectly reasonable for you to ask your solicitor for an estimate of the costs involved.

Mike