Could be.
One thing I was wondering about in respect of the cash. One early not unreasonable suggestion was that she may have been paying the builders in cash. That's been discounted but can anyone be sure? If you were one of the builders and had been being paid in cash would you come forward or admit it? Knowing the likely consequences from HMRC etc. As its a murder enquiry you'd hope they would but I'm not sure everyone would.
I think she was quite meticulous in terms of record keeping, so I'm presuming she would have demanded invoices/receipts for any significant work, regardless of whether she paid by cheque or cash. You never know though.
Builders don't usually receive full payment until the work is finished. They get some of the money upfront, but the rest once the job is done. A bit like hitmen really!