How to get money back for hoilday let-down

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Over Easter, we booked 4 nights away in a UK cottage (the agency said to book privately with owners if less than a week, which is waht we did). On the 3rd day there the boiler broke down first thing in the morning, so not heating and no hot water. We spent all morning trying to contact owner who failed - because it was Easter weekend - to find anyone who would come and fix it. We had to return home - snowing, no heating, two little ones.
Now, at the time, by phone she did offer us half the holiday cost as a refund - or, alternatively, another 4 nights out of season. We've asked her to email possible free dates three times now (in a note left at cottage; and over the phone twice) - to no avail. Please can anyone advise us what to do next? Presumably the agency won't help as it was a private areangement. To be honest, I think it was a bit miserly only offering us 50% refund as it compensated us in no way whatsoever for the disruption and disappointment for the holiday.

I'd write to the owners and copy the agency in, formally requesting the dates they have free, or alternatively suggesting your own dates f you want to take up the 4 nights option. Send it recorded delivery. I would have thought that the 4 nights should have been in season though since Easter is a peak holiday time? I suppose they may have sold out of the peak dates though.

well, their argument was that over the peak summer season, they rent out their cottage by the week, not just for four days, and that if we wanted a week over summer then we'd have to pay the difference, which I guess is reasonable..isn't it?

Would that be an option for you to pay for 3 nights and make it a week then?

yes i think so..which might make it worth it for a summer's holiday...well at least that was what she said..ie. would have to look at the difference and for us to pay that,tho I suspect a week's high summer will be costly.
If she refuses to honour what she proposed, do you think there is anything else I could do, besides notifying the agency - who'd say it was a private arrangment, I think - to make her give us the refund?