Hi Royal,
I know it is incredibly frustrating waiting for these things to be processed, and it is hard to understand how it can take so long, but 18 months is not unusual for a probate, even where there is no house to sell. Bear in mind that the solicitor has to wait on the Revenue and Probate offices to go through the motions and this takes time.
As for interest, the solicitor may keep some of it to meet administration costs. They can keep up to .75% and other than that all interest on monies held for more than 3 months is paid to the client.
Good luck.
Kate.