I don't see how you can be a major taxpayer if you're on a visitor visa but whatever...
December: can't comment on how late that is; from late December, that does NOT seem that far out of line with others but I'm not an expert. Check the tracker, compare to other files.
MPs office: you call or write their office. They have staff. You explain what your situation is and you'd like help getting info.
If you've paid an agent, you can surely push them to do more. But they probably won't (if like most agents, it seems). You can in the meantime get copies of EVERYTHING filed and double-check that the application was complete. Having an agent is no guarantee it was done properly.