How long was the previous shared care arrangement in place for. With a Fri-Mon and 2 o/n's per week this is basically shared care.
Do the children attend nursery?
Where would you normally collect/drop-off the children (e.g. was it from nursery or from the ex's).
As the children were benefiting, particularly in light of the children's care being somewhat neglected (have you proof of this btw, has it been raised with GP?) then the ex would need to have some concrete reasons for changing this status quo. The children must be wondering what on earth has happened to their routine - this cannot be good for them I would imagine.
If the ex's reasons for unilaterally trying to alter this arrangement are financial then that is NOT a child-centric reason.
Contact and money are not linked - in law at least, however, in practice many parents make this link as less o/n's at one parent = less money... totally wrong.
Does dad have PR? If he is named as father on the birth certificate then he automatically does.
There is nothing actually to stop dad from collecting the children from school (if that is what he used to do) and re-instating the previous schedule. This is something to think about.
As there are no court orders in place, there is nothing to prevent dad re-instating the shared care arrangement that have presumably been arrived at by consent.
The ex has no power or authority to unilaterally change the childrens parenting arrangements and disrupt their routine and stability. Possession is 9/10'ths of the law.
So you have some choices to make.
I would also advise you first to hook up with your local branch of Families Need Fathers (I'm the branch chair of my local branch) "www.fnf.org.uk"
You would normally be advised to propose family mediation to the ex. You can sort all this out yourself and find the local branch of Mediation to the ex. Find out how long their waiting list is, how much Mediation costs (if the ex is eligible for legal aid, she would have to go to Mediation first anyway).
Dad should do all of this, make an appt for himself first and go to it. Mediation should then write to the ex inviting her to attend.
Write to the ex stating the previous parenting arrangement and how long it had been in place. State that the children benefited from this and for the stability and routine of the children, in their best interests, this should be reinstated.
You have not agreed to this unilateral change of parenting arrangement and hope that you can both work together constructively for the child's best interests. To this end you propose Family Mediation to discuss parenting matters, you have made enquiries, they are available for Mediation on XYZ days and you extend the invitation for the ex to attend.
Ask the ex to respond that she is willing to attend Mediation, request that she responds within 2 weeks or you will have no other option but to make an appplication to Court so that an independent agency can make a decision as you both obviously cannot agree on matters.
In the meantime get along to your local FNF meeting or have a chat on the helpline/contact the local branch chair.
If you do end up making an application to Court it would be on a C100 form and I would imagine you would be applying for a Shared Residence Order.
The cost for a C100 is £175, there is no need to use a solicitor - you can do most of the work yourself (employing a solicitor later on if need be).
On the other hand you could look into using the help of a McKenzie friend. Some charge for this, others only ask for their costs to be covered. Some McKenzie's will also accompany you to Court and help out. Most McKenzie's will be able to prepare and help out with statements etc.
Keep a diary and write everything in it, note all that you have said here