First... consider doing things that will occupy their minds ! Kongs stuffed with peanut butter and frozen .... they take time to work through. There are a zillion things you can do to help them in this way. Toys that make them think etc....

Also.. consider constructing a proper fenced and floored dog run. The landlord isnt being picky... he wants his property in decent order.

Springer Spaniels are intense working dogs... (when you did your research before getting them you will have realised that) they cant just be 'left' they need occupation and interest. You are also going to have to put some time and re training in . This is a habit they have obviously got used to be able to doing (you said said they have been destroying garden stuff).

Start from square one with training... add alot of exercise.... ALOT ! Tired dogs are happy Furs !

You can turn this around !

Good Luck !