This thread has been an interesting read.
I'm an employment lawyer and whilst I do understand how inconvenient it can be for employers, particularly small employers, when an employee takes maternity leave, it has been the law for many years that you cannot take pregnancy or maternity leave into account in making employment decisions. That is the case even if you're hiring for a temporary contract and the employee will be away for much of it.
It is not the case, however, that employers have to bear the full cost of maternity pay. If only statutory maternity pay is paid, 92% can be reclaimed from the government by all employers. And small employers (those with class 1 NIC contributions - employer and employee - totalling £45,000 or less) can recover not only the full cost of SMP, but also an additional 3% compensation payment, which should cover at least some of their additional recruitment costs. See here for more info:
www.hmrc.gov.uk/manuals/spmmanual/spm21315.htm
As I said, I do understand the employer's predicament - but I also think it is disingenuous to say that, by hiding their pregnancies at interview, women make things harder for their sex generally. Women would not hide their pregnancy at interview if they could be sure that employers would act in a non-discriminatory way. As other have said above, pregnancy is a protected characteristic under the Equality Act because only women can become pregnant. It is not the same as long-term illness, or the desire to take a sabbatical, which affect both sexes equally. The position of women in the workplace has been precarious for many, many years because of purely biological factors. The law is there to try to change that.
OP, I wish you the best of luck in your interview. You are not obliged to reveal your pregnancy at interview. You deserve the chance to go for the job on an equal footing to everyone else. Although legally it makes no difference, the fact that the job is for a local authority means that they are more likely to have sophisticated maternity cover arrangements, so if you do get the job, I suspect they will be able to handle your pregnancy and maternity leave just fine.