I’m no expert and certainly unqualified but in the year when we came out of recession, to pay in £1350 but only see a £350 increase doesn’t seem right. Is that an up to date statement, or have you just got round to looking at it and the data is months old?
Second what was said about talking to the mortgage co (or an IFA). Don’t just wait until the end date, get the info from the endowment and a realistic projection on today’s rates what your shortfall is and then see what the mortgage co will do for you. You might be able to extend your term, or take a small short term mortgage to cover the projected shortfall. I don’t know your circumstances but £1350 a year = £115 a month roughly which is dead cheap, hopefully you have some extra per month you can throw at it.
But most of all, don’t panic, speak to someone asap and sort it. If you’re a good payer over the last 17 years I think highly unlikely they’d want to screw you over now.