Rent reductiion - no heating !

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Tenants viewed our rental property 3 weeks ago and we agreed a moving in date of the 1st of December. Since thier viewing the boiler has broken down (s days ago) and our usual plumber is struggling to fit us in to replace the heating system within the next 10 days. I have explained this to the prospective tenants and they are happy to delay moving in until the 10th of December as this is when there old AST runs out. They have said 'worse case' they would move in without heating (which i would prefer if they didnt) but have suggested a reduction in rent! There is working gas fire in lounge, hot water on immersion system and working shower.

Do I need to reduce rent as effectively they have agreed to take on the property as it is?

If I do reduce the £650 per month rent for a 3 bed property, what is a reasonable amount per day to reduce rent by?