>>>We don't live over there we only use it as a holiday home
So HMRC will regard you as domiciled in the UK and its out inheritance tax laws that will apply:
https://www.gov.uk/inheritance-tax/inheritance-tax-when-someone-living-outside-the-uk-dies
(Although that link refers to people living overseas it also covers the rules for those who don't).
Hence UK IHT will be applied where relevant. However IHT is not payable on anything left to a spouse (or civil partner) where that spouse/partner is permanently resident in the UK.
So, if you alone were to die then (I assume, since you've not stated otherwise, that) the whole of your Spanish property would pass to your husband. Upon his death your
combined IHT exemption (i.e. TWO lots of £325,000 [at current rates]) would be applied, so IHT would only be payable on that part of your husband's estate (if any) exceeding £650,000.
See here for full information about IHT:
http://www.hmrc.gov.uk/inheritancetax/understand/index.htm
However I'm concerned to read the phrase 'our Spanish will'. I sincerely hope that you've each only got ONE will; if you've got two then either (depending upon the wording) the later one will revoke the earlier one anyway or you'll risk the surviving partner (or later your son) having a large part of their inheritance gobbled up in legal fees trying to sort everything out!