D&C raises a good point but under dynamic coding i think hmrc are processing more adjustments in the following year than they used to so i wouldn't presume that 17/18 tax is not be collected now.
I would recommend ringing hmrc and speaking to an advisor and asking them to confirm the numbers if its not clear from looking at the 18/19 code if thats collecting 17/18 tax.
Also give them the correct figures ye 5/4/18 and ask them to adjust first then see how that flows through to coding, if unsure why coding adjusts are there ask for clarification. Give them an expected ye 5/4/19 interest received figure too
Ref coding adjusts is all down the techinical nitty gritty of whether the adjustments this year are
(1) for actual underpaid tax from ye 5/4/18 as per ye 5/4/2018 tax calculation issued.
(2) for estimated interest received current year so that code this year does its job
(3) an extra adjustment to your code to collect for (1) or (2) or (1) and (2) based on the fact that your code for the earlier part for this year was not previously collecting the taxz.
Unless you know for certain its 1 in your tax code ye 5/4/2019 it may be that you may still owe the tax come the year end for that prior year, even if 1 (and 3) is in your tax code its stil possible you may run short again this year if any dynamic coding adjustment messes up - they are prone to mess up. Potentially though you are paying two years tax two months !!!
To add insult to injury hmrc have recently been having issues for sa clients where incorrect underpaid amounts have appeared in the ye tax calcs - not sure if this has stretched to paye or not.
In some respects if you can nip it in the bud lowering interest received total the problem will mitigate the issue.
however the change in the code will take far more out of my PAYE payments than they need to cover the amount of unpaid tax they have stated I owe.
This is to be expected under dynamic coding if they are also adjusting for similar issues this tax year too.
Note you may need to speak to more than one advisor to get a ninja one but generally most of them should be able to hopefully revise interest figures presuming they are "generic estimates" and get back to where you should be in that you understand your code.
In summary pick up the phone and hmrc will hopefully enlighten you.