Mortgage and HTB

xssc32x

Hi, looking for advice on incorporating HTB with mortgage.
my 5 year fixed mortgage is up for renewal at the end of June, and also want to incorporate the HTB loan that I took out. 

I am in conversation with a mortgage broker who looks to have secured a deal via Halifax, this is yet to be confirmed but it’s all looking good and the offer is based on a desktop valuation of the property. From my understanding the HTB element requires another independent valuation of the property to determine how much I owe total based on 20% on the properties increase in price. Would like some advice on when I need to get this valuation and relevant forms completed and sent back to HTB ( also appointing a solicitor to oversee paperwork). With new mortgage needed at end of June do I need to wait until start of April to ensure the HTB valuation is within the 3 months prior to new mortgage? Also how long will the new deal mortgage be ‘held’ for awaiting the HTB element to be finalised?
any help/advice/tips would be really appreciated 
cheers
si