Database (re)design help

Hello,

I started a small database many years ago in access that kept track of some
patient details - I work in a GP surgery - and their recalls. This has grown
itself out to holding certain data and results of all patients and keeping
track of their recalls. Doesn't sound very different but I think the old
structure is very cumbersome to work with nowadays.

What I have:
A MainTable that holds patient details (contacts, test results, some other
bits of info). All the details get updated every month with the latest
information through append and update queries from imported tables.

An InvitationsTable that holds the PatientID from MainTable and the
following fields:
TestID - A lookup field where I enter the name of the test the patient needs
to be sent for
SendIn - the number of the month when the patient needs to be sent
CheckIn - the number of the month when I check whether the patient has
attended the test (generally the month following SendIn)
1stSent
2ndSent
3rdSent
4thSent
LastSent - these hold the number of the month when the patient was sent an
invitation. And this is my headache. When this setup was introduced the idea
was that I remove patients from the InvitationsTable after 3 non-attendance
but some time ago the policy has changed. In the LastSent now I just enter
the month I'm sending the patient but this way loads of data may get lost if
the patient is a particularly lazy one, and we have quite a few of that. Also
my left thumb is tired of going to the latest Sent field. I'm really hoping
there is an easier way to do this.

What I need is a setup which supports an easy-entry form for invitations but
also makes it possible for me to query if a patient has more than 3
invitations in the previous 3 months, so we can recall them.

I have had discussions on this already but no conclusion. One suggestion was
to keep a record of all invitations ever done - at present I delete the
invitations data if the patient attended or has been excluded from being
invited.

I have tried one-to-many relationships with three tables - one for data,
one for date and one for the name of the bloodtests, but in form view it has
not been workable at all.

I am moving on to Access 2007 from 2003, am not a total beginner and quite
comfortably find my way around tables, queries and forms, but I do find some
of the more complex processes... well, a bit complex. Thank you very much for
your help if any.