Legally you're entitled to a total of 39 weeks paid maternity leave. Looks like they will increase your maternity pay for the first 14 weeks and then the remaining 25 weeks at statutory rate. But the way its written seems to contradict this so I'd question it with them as they may need to update their policy. As long as you meet the eligibility criteria that is. Check out the gov.uk website for info on statutory maternity pay as they can't offer you less than this.
First 6 weeks full pay, next 8 weeks is half pay, the remaining would be statutory only, which is 184 per week.
It looks to me like they will give you an extra supplement for the first 6 weeks to raise it from 90% pay to full pay, the 8 weeks is saying it will give an extra supplement for raise it to 50% pay but as you say the paragraph above that says you get 20 weeks at whatever the agreed rate is. Do I guess it depends on what they have as the agreed rate for those weeks