ImportXML fails - alternative? - google-apps-script

Question

I've created a spreadsheet to pull in data from two river gauges near my house to try and follow their relationship to the water level as it passes us, (it's an old mill) since more and more the country seems to be flooding!
Basically the sheet works perfectly but not when the spreadsheet isn't open/when I close the laptop. I thought openById and getSheetByName would solve this but I'm having no luck.
The ImportXML commands that I construct just return N/A when the sheet is not open. The script is on a timed trigger every 15mins. I have another basic script that rewrites the contents of the cells as values so that there is only one row of ImportXML commands at a time.This script is also on a timed trigger.
I've pasted the scripts below - apologies if they're not very eloquent, these are my first attempts!
function fetchData() {
var sheet = SpreadsheetApp.openById("0AgG7jQSg2ljFdGNpbFRjUTA1LXJwdmtPcmxkQnZpUHc").getSheetByName("River Levels");
var bredURL = "http://www.environment-agency.gov.uk/homeandleisure/floods/riverlevels/120743.aspx?stationId=4004";
var belURL = "http://www.environment-agency.gov.uk/homeandleisure/floods/riverlevels/120743.aspx?stationId=4002";
var ssBredLevel = "substring((//div[#class='plain_text']/p)[2],35,4)";
var ssBelLevel = "substring((//div[#class='plain_text']/p)[2],31,4)";
var ssTime = "substring(//div[#class='plain_text']/p[2],34,5)";
var ssDate = "substring(//div[#class='plain_text']/p[2],43,10)";
var commandBredLevel = "ImportXML(\"" + bredURL + "\",\"" + ssBredLevel + "\")"; // build ImportXML() command
var commandBredTime = "ImportXML(\"" + bredURL + "\",\"" + ssTime + "\")"; // build ImportXML() command
var commandBredDate = "ImportXML(\"" + bredURL + "\",\"" + ssDate + "\")"; // build ImportXML() command
var commandBelLevel = "ImportXML(\"" + belURL + "\",\"" + ssBelLevel + "\")"; // build ImportXML() command
var commandBelTime = "ImportXML(\"" + belURL + "\",\"" + ssTime + "\")"; // build ImportXML() command
var commandBelDate = "ImportXML(\"" + belURL + "\",\"" + ssDate + "\")"; // build ImportXML() command
sheet.appendRow(['Bredwardine',' ',' ',' ',' ','Belmont',' ',' ',' ',' ','=now()']); // append row
var lastrow = sheet.getLastRow(); // define last row
var bredLevel = sheet.getRange(lastrow, 2); // define ImportXML cell
var bredTime = sheet.getRange(lastrow, 3); // define ImportXML cell
var bredDate = sheet.getRange(lastrow, 4); // define ImportXML cell
var belLevel = sheet.getRange(lastrow, 7); // define ImportXML cell
var belTime = sheet.getRange(lastrow, 8); // define ImportXML cell
var belDate = sheet.getRange(lastrow, 9); // define ImportXML cell
bredLevel.setFormula(commandBredLevel);
bredTime.setFormula(commandBredTime);
bredDate.setFormula(commandBredDate);
belLevel.setFormula(commandBelLevel);
belTime.setFormula(commandBelTime);
belDate.setFormula(commandBelDate);
writeFormats();
}
function writeRows() {
var sheet = SpreadsheetApp.openById("0AgG7jQSg2ljFdGNpbFRjUTA1LXJwdmtPcmxkQnZpUHc").getSheetByName("River Levels");
var range = sheet.getRange(1, 1, sheet.getLastRow(), sheet.getLastColumn());
range.setValues(range.getValues());
};
function writeFormats() {
var sheet = SpreadsheetApp.openById("0AgG7jQSg2ljFdGNpbFRjUTA1LXJwdmtPcmxkQnZpUHc").getSheetByName("River Levels");
var lastcol = sheet.getLastColumn(); // define width of sheet
var lastrow = sheet.getLastRow(); // define last row
var penrow = lastrow-1; // define penultimate row
var formatting = sheet.getRange(penrow,1,1,lastcol); // fetch formatting from penultimate row
formatting.copyFormatToRange(sheet,1,lastcol,lastrow,lastrow); // paste formatting to last row
};