Can I clear some stuff up about the chronology of this:
- You went on maternity leave;
-After six months, you weren't happy that arrangements had been made for your illness/disability, so you stayed on maternity leave. Did you tell anyone at the time that this was why you were staying off?
-Presumably, the 12 months of maternity leave ended (otherwise you wouldn't have been a 'no show' at work). What happened then, were you on sick leave? If so, how long was this for and did you have a doctors' note?
- You agreed with occ health that they would set up a meeting via your mum. However, the message went awry somewhere and two meetings were arranged at work?
-Did you receive notification of these meetings and, if so, did you or your mum contact the company to let them know what had been agreed? If you simply ignored the letters then I think that this is relevant as, although the company is still potentially at fault for poor communication, there is less evidence that they acted with deliberate malice.
-What happened after you found out about the meetings?
In terms of compensation, bear in mind that it will be based on your loss of earnings. The six months' additional maternity leave will be very relevant and you should discuss with your solicitor what evidence there is that the extension was to do with your employer's behaviour. Were there emails to request adjustments, notification of illness? Flexible working requests? The more evidence, the more chance you have of including this as a loss. If the tribunal thinks you would have taken the leave anyway, no loss and therefore no compensation for this bit.
Then think about how much this offer represents in NET pay. Some or all of it is probably tax free, but awards at tribunal are based on net loss of earnings, not gross pay. Does it roughly correlate to how long you think you need to find a new job? If not, why are they discounting? Are there parts of your claim that are weak?
On the tactics, if the hearing is not until June, what other steps need to be done soon in preparation for the hearing? If there are not big outlays in legal fees coming up imminently (eg. witness statements), it is likely that your employer is putting the deadline in place to encourage you to get a move on, not as an absolute drop dead. Even if they don't agree to extend the deadline, if you came back after it had expired and offered exactly the same amount as your own settlement proposal, they would probably take it.
In terms of saying something will be the only offer, well that's just negotiation I'm afraid . It's like if you're buying a house or a second hand car. You often say, "well my best offer is X" of "well all I can offer is X", even if you'd go to a bit more. It's all a game of bluff and all is fair in love and war! One thing I would say is that first offers are rarely final offers, there is almost always a teeny bit extra in the budget for negotiation.