[SalesForce] Error: Incorrect parameter type for operator ‘+’. Expected Text, received Number

In your case I would make a wrapper class that holds the info you want. Store the wrapper in the map, and then reference the wrapper.

public map<String, List<WrapperClass>> mapFYToList {get;private set;}

public SDGProgrammeController()
{   
 mapFYToList = new map<String, List<WrapperClass>>();

 for(Site__c r : [SELECT StoreNo__c, Probability__c, Name, JS_Region__c,JS_Zone__c, Address__c, Postcode__c, 
                            (SELECT MeetingDate__c,RecordType.name,Total_Sales_Area_Uplift__c, TU_Sales_Area_Uplift__c, TUSalesAreaPre__c, 
                                    TUSalesAreaPost__c, SalesAreaPreExcCO__c, SalesAreaPostExcCO__c, NonFoodSalesAreaUplift__c, NonFoodSalesAreaPre__c, 
                                    NonFoodSalesAreaPost__c, GMSalesAreaUplift__c, GMSalesAreaPre__c, GMSalesAreaPost__c, FoodSalesAreaUplift__c, 
                                    FoodSalesAreaPre__c, FoodSalesAreaPost__c 
                                FROM Financial__r 
                                WHERE MeetingDate__c != null AND Recordtype.Name IN('CCM','CCM Update','IB')
                                ORDER BY MeetingDate__c desc
                                LIMIT 1),
                            (SELECT Launch_date__c,Start_On_Site__c, Status__c,Store__c,ProjectName__c,Investment_Type__c,ProjectManager__c, 
                                    MainContractor__c,RetailHandover__c,Closure__c,Launch_FY__c,Launch_Period__c, Launch_Quarter__c 
                                 FROM Investment_Programs__r 
                                 WHERE Status__c IN('Live','Feasibility','Completed') AND RecordType.Name ='New Store' )
                    FROM Site__c 
                    WHERE Recordtype.name = 'Supermarket' AND Probability__c IN('Open','Certain','Possible','Probable')
                        AND Id IN(SELECT Site__c FROM Financial__c WHERE MeetingDate__c != null AND Recordtype.Name IN('CCM','CCM Update','IB'))
                        AND Id IN(SELECT Site__c FROM InvestmentProgram__c WHERE Status__c IN('Live','Feasibility','Completed') AND RecordType.Name ='New Store') 
                        ORDER BY Launch_Date__c asc LIMIT 999])
{
    if(mapFYToList.get(r.Investment_Programs__r[0].Launch_FY__c) == null)
    {
        mapFyToList.put(r.Investment_Programs__r[0].Launch_FY__c, new List<WrapperClass>{new WrapperClass(r)};
    }
    else
    {
        mapFyToList.get(r.Investment_Programs__r[0].Launch_FY__c).add(new WrapperClass(r));
    }
    system.debug(mapFyToList);
}


}
public List<String> getKeys()
{
    List<String> keys = new List<String>();
    keys.AddAll(mapFYToList.KeySet());
    keys.sort();
    return keys;
}

public class WrapperClass
{
    String name {get; set;}
    String postCode {get; set;}
    Stirng status {get; set;}
    Integer probability {get; set;}

    public WrapperClass(Site__c site)
    {
        name = site.Name;
        postCode = site.Postcode__c;
        status = site.Investment_Programs__r[0].Status__c;
        probability = site.Probability__c;
    }
}
}

And then in your VF Page

<apex:repeat value="{!keys}" var="fy">
  <apex:pageBlockTable value="{!mapFYToList[fy]}" var="obj">
    <apex:column value="{!obj.name}"/>
    <apex:column value="{!obj.postCode}"/>
    <apex:column value="{!obj.status}"/>
    <apex:column value="{!obj.probability}"/>
  </apex:pageBLockTable>
</apex:repeat>

I was able to get a very similar to piece of code to yours to work - but I found that I couldn't even compile the page using outputField - You should only use outputField for values that are directly tied to a sObject value.

Change to outputText and also verify that wrapRec.T_Price__c is in fact a number value.

Failing that, try wrapping the value to be added in a VALUE() function.

EDIT Here is the code I used. I used a list of accounts in the controller.

<apex:pageBlock >
    <apex:pageBlockSection columns="1" collapsible="false" >
        <apex:pageBlockTable value="{!acctWrapper}" var="acct">
            <apex:column value="{!acct.acctId}"/>
            <apex:column value="{!acct.name}"/>

            <apex:column >
                <apex:variable var="total1" value="{!total1 + acct.num}" />
                <apex:outputText value="{!total1}" />
            </apex:column>

            <apex:column >
                <apex:variable var="total2" value="{!total2 + acct.num}" />
                <apex:outputText value="{!total2}" />
            </apex:column>

        </apex:pageBlockTable>
    </apex:pageBlockSection>
</apex:pageBlock>

Here is the wrapper code:

public class Wrapper {
    public String name {get; set;}
    public Id acctId {get; set;}
    public Integer num {get; set;}


    public Wrapper(String name, Id acctId, Integer num) {
      this.name = name;
      this.acctId = acctId;
      this.num = num;
    }
}

And here is where I build the wrapper list:

//defined at top of class
public Wrapper[] acctWrapper {get; set;}


accounts = [SELECT Name, Id From Account LIMIT 20];
acctWrapper = new Wrapper[]{};
Integer count = 0;
for (Account a : accounts){
    acctWrapper.add(new Wrapper(a.Name, a.Id, count));
    count+=1;
}