Renouncing both Executors and replacing with Administrators

[AP9009]

Hi - I hope you can help on the best course of action for us to resolve my dads estate as soon as we can.

My dad passed away 3 weeks ago and his will named two executors (his brother and sister), both of whom are alive. Me and my two sisters are the sole beneficiaries, receiving a third each. I suspect the estate is between £400 and £500k. It will be a house (£300k), pensions, investments etc. nothing out of the ordinary. The reality is that me and my two sisters will be doing all the admin here. I think the best course of action is:

1. Obtain original will (the Executors have already requested this from the solicitor)
2. Get current Executors to complete PA15 to renounce their role (they are happy to do this if we think its best course of action)
3. Us to fill in IHT form online (our mum died in 1996 and left everything to our dad, so we think the band will be increased, allowing us to use this form). We have her will (copy) but no evidence of the amounts (is this an issue?). Otherwise we can use IHT400 and extend to £500k as the house will pass to us.
4. Then we will fill in form PA1P - as there is a will and attach reference to the online IHT form, as well as the two PA15 forms. Me and my two sisters will apply to be administrators.

Does this look to be the right thing to do? I imagine not doing PA15 will result in many delays while my uncle and aunty deal with the estate if they got probate. They are knocking on a bit.

The issue i can see here is gaining access to the accounts where assets or liabilities may be. Will banks/pension funds etc, be able to release a figure to us for the purpose of the IHT calculation, or will they only deal with the current executors (as IHT has to be done before we apply to be administrators)?

Any advice is appreciated!

Cheers